CT600 Late Filing Penalties 2026: From £100 to £10,000+ Explained

Last updated: May 2026.

Missing your CT600 filing deadline is one of the most expensive mistakes a UK limited company director can make. HMRC's penalty regime is automatic, escalating, and entirely avoidable. Yet thousands of companies miss the deadline every year, often because they did not realise how quickly the costs stack up.

This guide covers every penalty, interest charge, and appeal route so you know exactly what you are facing, and what to do about it.

What Is the CT600 Filing Deadline?

Your CT600 Corporation Tax Return must be filed with HMRC within 12 months of the end of your company's accounting period.

If your accounting period ends on 31 March 2026, your CT600 is due by 31 March 2027. If it ends on 31 December 2025, the deadline is 31 December 2026.

This is separate from the payment deadline, which falls earlier. You must pay your corporation tax 9 months and 1 day after the accounting period ends. So the payment is due before the return itself.

For a deeper overview of all the key dates, see our corporation tax deadline guide.

Why Does This Matter?

The filing and payment deadlines are independent. You can be penalised for filing late even if you have already paid your tax. And you can be charged interest on late payment even if your return is filed on time. Understanding both is essential.

CT600 Late Filing Penalties: The Full Breakdown

HMRC applies automatic penalties for late CT600 returns. These are fixed charges that apply regardless of your tax liability, and they escalate the longer the return remains outstanding.

Here is the complete penalty schedule:

How the Penalties Work Step by Step

Day 1. The moment your CT600 is one day overdue, HMRC issues an automatic £100 penalty. There is no grace period, no warning letter first, and no discretion. The penalty is generated by HMRC's systems automatically.

3 months. If you still have not filed after three months, a second £100 penalty is added. At this point you owe £200 in penalties alone, regardless of your tax position.

6 months. The penalties become tax-based. HMRC charges 10% of the corporation tax that remains unpaid at the date the return was due. If 10% of your unpaid tax is less than £300, the minimum penalty is £300. So even if your company had zero tax to pay, you would owe at least £500 in total penalties (£100 + £100 + £300 minimum).

12 months. A further 10% of the unpaid tax is charged, again with a £300 minimum. Your total penalties are now at least £800 for a company with no tax liability, and significantly more if tax was outstanding.

The £100 Penalty Doubles for Repeat Offenders

If your company files its CT600 late three times in a row (three consecutive accounting periods), the initial £100 penalty increases to £500 for the third and subsequent late returns. The three-month penalty also rises to £500.

This means a company that has been late three years running faces:

For a repeat offender with no tax to pay, the minimum total after 12 months is £1,600 (£500 + £500 + £300 + £300).

What About Late Payment Interest?

Penalties and interest are separate charges. Even if your return is only slightly late, interest on any unpaid corporation tax starts accruing from the day after the payment deadline.

Current HMRC Interest Rates

As of 2026, the HMRC late payment interest rate is around 8.5% per annum. It is set at the Bank of England base rate plus 4 percentage points — the margin rose from 2.5 to 4 points in April 2025 — and changes whenever the base rate moves.

Interest is calculated daily on the outstanding amount and runs until the day the tax is paid in full. The longer you leave it, the more it costs.

HMRC also pays repayment interest if you overpay your tax, but the rate is significantly lower — around 3.5%, set at the base rate minus 1 percentage point with a 0.5% floor. The asymmetry between what HMRC charges you and what it pays you is deliberate. For the latest published figures, see our HMRC interest rates guide.

How Interest Stacks on Top of Penalties

This is the critical point many directors miss. The penalties above are charges for late filing. Interest is a charge for late payment. If you both file late and pay late, you face both.

The interest runs from the day after the payment deadline (9 months and 1 day after your accounting period) until the date HMRC receives your payment in full. It does not stop when you file your return. It only stops when the money arrives.

Worked Example: Filing 7 Months Late with £5,000 Tax Due

Let us walk through a realistic scenario to show exactly how the costs add up.

Company: Acme Solutions Ltd Accounting period ends: 31 March 2026 Corporation tax due: £5,000 Payment deadline: 1 January 2027 (9 months and 1 day) Filing deadline: 31 March 2027 (12 months) Actual filing date: 31 October 2027 (7 months late) Actual payment date: 31 October 2027 (10 months after payment deadline)

Penalty Calculation

The company filed before the 12-month mark, so the fourth penalty does not apply.

Interest Calculation

Interest runs from 2 January 2027 (day after payment deadline) to 31 October 2027 (payment date). That is approximately 303 days.

Interest = £5,000 x 8.5% x (303/365) = £352.74

Total Cost of Filing 7 Months Late

The company pays £1,052.74 more than it would have paid by filing and paying on time. That is a 21% surcharge on the original tax bill.

Worked Example: Filing 14 Months Late with £12,000 Tax Due

Here is a worse scenario to illustrate how the 12-month penalty bites.

Company: Beta Consulting Ltd Accounting period ends: 30 June 2025 Corporation tax due: £12,000 Payment deadline: 1 April 2026 Filing deadline: 30 June 2026 Actual filing and payment date: 31 August 2027 (14 months late)

Penalty Calculation

Interest Calculation

Interest runs from 2 April 2026 to 31 August 2027. That is approximately 517 days.

Interest = £12,000 x 8.5% x (517/365) = £1,443.95

Total Cost

That is an extra £4,043.95, or roughly 34% on top of the original tax bill. For a small company, that is a serious amount of money.

Worked Example: The Repeat Offender

This example shows how much more expensive penalties become once the £100 fixed charges have doubled to £500.

Company: Delta Trading Ltd History: Filed its CT600 late in the two previous accounting periods Accounting period ends: 31 March 2026 Corporation tax due: £8,000 Filing deadline: 31 March 2027 Actual filing and payment date: 30 October 2027 (7 months late)

Because this is the third consecutive late return, the two fixed penalties are charged at £500 each rather than £100.

Penalty Calculation

Interest Calculation

Interest runs from 2 January 2027 (day after the payment deadline) to 30 October 2027 — about 301 days.

Interest = £8,000 × 8.5% × (301/365) = £560.77

Total Cost

The repeat-offender doubling adds £800 (two £400 increases) on top of what a first-time late filer would pay. If the same company had also passed the 12-month mark, a further £800 tax-geared penalty would apply, taking total penalties to £2,600. Persistent lateness is the single most expensive habit a director can fall into — and breaking the cycle requires filing one return on time, which resets the count.

Worked Example: Filed On Time, Paid Late

It is worth seeing the opposite case — what happens when the return is in on time but the company simply cannot pay the bill by the deadline.

Company: Epsilon Ltd Accounting period ends: 31 March 2026 Corporation tax due: £15,000 Payment deadline: 1 January 2027 Filing deadline: 31 March 2027 CT600 filed: 20 March 2027 (on time) Tax paid: 1 June 2027 (about 150 days after the payment deadline)

The Result

The entire extra cost is interest. Not a single fixed penalty applies, because the £100, £100 and tax-geared penalties are triggered by a late return, not a late payment. This is the most important practical lesson in the whole penalty regime: if cash is tight, file your CT600 on time regardless. Filing costs nothing and protects you from every fixed penalty. Pay what you can by the deadline, arrange Time to Pay for the rest, and accept the interest as the only consequence.

What If Your Company Has No Tax to Pay?

This catches many directors off guard. Even if your company made a loss and owes zero corporation tax, you must still file the CT600 return. HMRC penalties for late filing apply regardless of your tax position.

With no tax to pay, the penalties look like this:

There is no interest because there is no outstanding tax. But £800 in penalties for a company that owed nothing is a painful and entirely unnecessary cost.

This is particularly relevant for dormant companies. If your company is registered but not trading, you still need to file a CT600 unless you have notified HMRC that the company is dormant. Many directors assume dormant means "nothing to do." It does not.

How Do HMRC Late Filing Penalties Differ from Late Payment Penalties?

This distinction confuses many directors. There are two separate penalty regimes:

Late filing penalties are charged when your CT600 return is submitted after the 12-month deadline. These are the fixed and percentage penalties described above.

Late payment interest is charged when your corporation tax is paid after the 9 months and 1 day payment deadline. This is a daily interest charge at 8.5%.

You can be charged one without the other. For example:

• Filed on time, paid late: No filing penalties. Interest charged on the late payment.
• Filed late, paid on time: Filing penalties apply. No interest (tax was already paid).
• Filed late, paid late: Both filing penalties and interest apply.

The worst outcome is filing and paying late simultaneously, because both charges stack.

Can HMRC Charge Additional Penalties for Inaccurate Returns?

Yes. If your CT600 contains inaccuracies, HMRC can charge a separate penalty on top of the late filing penalties. The inaccuracy penalty ranges from 0% to 100% of the additional tax due, depending on the nature of the error:

These penalties are assessed separately and are in addition to any late filing penalties. However, they only apply to inaccurate returns, not simply late ones.

What Counts as a "Reasonable Excuse" for Late Filing?

HMRC will cancel late filing penalties if you can demonstrate a reasonable excuse for missing the deadline. But their bar is high, and they reject the vast majority of appeals.

Excuses HMRC Typically Accepts

• Serious illness or life-threatening condition that prevented you from dealing with your tax affairs. You will need medical evidence.
• Death of a partner or close family member shortly before the deadline.
• Fire, flood, or natural disaster that destroyed your records or prevented access to them.
• HMRC's own service issues, such as their online filing system being down during the final days before the deadline.
• Postal delays if you filed a paper return and can prove it was posted in good time (increasingly rare since most filing is online).
• Unexpected hospital stay that coincided with the deadline period.

Excuses HMRC Typically Rejects

• "My accountant did not file on time." You are legally responsible for your company's filing obligations. An accountant acting on your behalf does not transfer that responsibility. HMRC's view is that you should have checked.
• "I did not know the deadline." Ignorance of the law is not an excuse.
• "I did not receive a reminder." HMRC is not obligated to remind you.
• "My records were with my accountant." You should have ensured they had everything they needed with enough time.
• "I was too busy." Running a business does not exempt you from filing obligations.
• "My software was not working." You should have found an alternative method.
• "I could not afford an accountant." HMRC expects you to file the return yourself if you cannot afford professional help.

The Key Test

HMRC applies a two-part test:

1. Was there a reasonable excuse that prevented you from filing on time?
2. Did you file the return without unreasonable delay once the excuse no longer applied?

Both parts must be satisfied. If your illness lasted two weeks but you waited another three months after recovering, HMRC will reject the appeal.

How to Appeal a CT600 Late Filing Penalty

If you believe you have a reasonable excuse, or if HMRC has made an error (for example, penalising you for a return that was filed on time), you can appeal.

Step 1: Appeal Online or by Post

You can appeal online through your HMRC business tax account, or by writing to:

Corporation Tax Services HM Revenue and Customs BX9 1AX

Step 2: Provide Evidence

Include the reason for the late filing, the dates relevant to your excuse, and supporting evidence (medical certificates, hospital letters, screenshots of HMRC system errors, proof of posting).

Step 3: Wait for a Decision

HMRC will review your appeal and respond in writing. This can take several weeks.

Step 4: Escalate if Needed

If HMRC rejects your appeal, you can request a statutory review by a different HMRC officer. If that fails, you can appeal to the First-tier Tribunal (Tax Chamber), which is independent of HMRC.

The tribunal route is free to apply for but takes time. For penalties under £1,000, most directors accept the penalty rather than pursue a tribunal hearing. For larger amounts, it can be worth it.

Time Limits for Appeals

You must appeal within 30 days of the penalty notice. If you miss this window, you can still apply for a late appeal, but you will need to explain why you did not appeal sooner.

Special Reduction: When HMRC Can Reduce a Penalty Outside the Normal Rules

Separate from the reasonable-excuse route, HMRC has a statutory power to reduce a penalty because of special circumstances. This is a narrower and less-used route, but it is worth knowing it exists.

"Special circumstances" is not precisely defined in law, but it means something genuinely uncommon or exceptional — something that makes the standard penalty disproportionate or unfair in your specific case. It is not the same as a reasonable excuse: a reasonable excuse cancels the penalty entirely because you were prevented from filing, whereas a special reduction acknowledges the penalty is technically due but reduces it because the circumstances are exceptional.

Two things are explicitly not special circumstances:

• An inability to pay the penalty or the tax.
• The fact that one event has effectively been penalised twice.

If you think special circumstances apply, raise them in your appeal or your request for a statutory review, and explain clearly why your situation is out of the ordinary. The First-tier Tribunal can also consider special circumstances if HMRC's decision on the point was flawed. In practice, special reduction succeeds rarely — but for an unusual case it is a legitimate argument to put forward alongside, or instead of, a reasonable-excuse claim.

What to Do in the First 30 Days After Missing the Deadline

If you have just realised your CT600 is overdue, the next few weeks matter. Acting quickly limits the damage. Here is a practical sequence.

Days 1–3: File, or commit to a filing date. The single most valuable action is to get the return in. Every penalty threshold after day one is time-based, so filing now stops the clock before the three-month and six-month charges land. If your accounts are not ready, file with your best estimated figures rather than waiting — an estimated return can be amended within 12 months of the filing deadline, and it stops the escalation.

Days 1–3: Pay the tax, or pay what you can. Interest is accruing daily at around 8.5%. Pay the full liability if you can. If you cannot, pay as much as possible to reduce the interest base, and move to the next step.

Days 3–7: Arrange Time to Pay if you cannot pay in full. Call HMRC's payment support line on 0300 200 3835. A Time to Pay arrangement spreads the balance over an agreed period — interest still runs, but you avoid enforcement action. HMRC is far more cooperative with a company that comes forward early.

Days 7–14: Gather evidence if you have a reasonable excuse. If something genuinely outside your control caused the delay — serious illness, bereavement, an HMRC system failure — collect the dates and documentary evidence now, while it is fresh. You will need it for an appeal.

Days 14–30: Appeal any penalty notice within 30 days. Penalty notices must be appealed within 30 days of their date. If you have a reasonable excuse or special circumstances, submit the appeal through your HMRC business tax account or in writing, with your evidence attached.

Throughout: Set up reminders so it never recurs. Diary next year's deadlines immediately. One on-time return also resets the repeat-offender count, so getting back on track has lasting value.

How to File Your CT600 Late

If you have missed the deadline, file as soon as possible. Every day you delay adds to the interest charges and brings you closer to the next penalty threshold.

Filing Online

Most companies file their CT600 through HMRC's online services or commercial software. You will need:

• Your company UTR (Unique Taxpayer Reference)
• Your company's financial accounts for the accounting period
• A tax computation showing how the corporation tax was calculated
• iXBRL-tagged accounts (most software generates these automatically)

Using an Accountant

If you are using an accountant, contact them immediately. Make clear that the return is overdue and ask for a specific date by which they will file it. Do not leave it open-ended.

Using AccountsOS

AccountsOS tracks your corporation tax deadlines automatically. When you connect your company, we sync your filing deadlines from Companies House and set up alerts so you never miss a date. If you are approaching a deadline, Finn (our AI assistant) will flag it in your dashboard and via email.

For the current corporation tax rates and thresholds, see our dedicated guide.

What If You Never File at All? HMRC Determinations

Penalties assume you eventually file. But what happens if you simply never submit the return? HMRC does not wait indefinitely. Once the filing deadline has passed, HMRC can issue a determination — its own assessment of the corporation tax it believes your company owes.

A determination has three features that make it dangerous:

1. It is HMRC's estimate, and it is rarely generous. HMRC bases the figure on whatever information it holds — previous returns, bank data, industry norms — and tends to estimate on the high side. You may be assessed for far more tax than you actually owe.

2. It cannot be appealed. Unlike a penalty, there is no reasonable-excuse route and no tribunal challenge against the determination figure itself. It is legally enforceable as though it were your own self-assessment, and HMRC can pursue collection — including debt enforcement and winding-up action — on the strength of it.

3. The only way to displace it is to file the actual return. Submitting your real CT600 replaces the determination with your genuine figures. You generally have to do this within three years of the filing deadline (or twelve months from the date of the determination, if later). Miss that window and HMRC's estimate stands permanently.

Crucially, a determination also feeds the tax-geared penalties. The 6-month and 12-month penalties are 10% of the tax unpaid at the filing date — and if you have not filed, HMRC uses the determined figure to calculate them. An inflated determination therefore produces inflated penalties. Filing, even late and even with estimated numbers, is always better than leaving HMRC to guess.

What Happens If You Ignore the Penalties?

Ignoring HMRC penalties does not make them go away. The consequences escalate:

1. Penalty notices. HMRC sends written notices for each penalty as it is applied.
2. Interest accrual. Interest continues to accrue on both the unpaid tax and the unpaid penalties.
3. Debt collection. HMRC will pass the debt to their debt management team, who will contact you by phone and letter.
4. County Court Judgment (CCJ). HMRC can apply for a CCJ against your company, which damages your credit rating and becomes public record.
5. Winding-up petition. For persistent non-payment, HMRC can petition to wind up your company. This is not an idle threat. HMRC is the single largest petitioner for compulsory company liquidations in the UK.
6. Director disqualification. In extreme cases, the Insolvency Service can investigate directors of companies that fail to meet their statutory obligations.

The message is clear: pay the penalty, file the return, and move on. Fighting it only makes sense if you genuinely have a reasonable excuse.

How Do Penalties Work for Your First Accounting Period?

Your company's first accounting period starts on the date of incorporation and typically ends on the accounting reference date set at Companies House (usually the last day of the month in which you incorporated, one year later).

For brand new companies, there is no adjustment to the penalty regime. The same deadlines and penalties apply from your very first return. Many first-time directors are caught out because they assume HMRC will give them extra time or a warning. They do not.

If your first accounting period is longer than 12 months (which it can be, up to 18 months), you may need to file two CT600 returns: one for the first 12 months and one for the remaining period. Both have their own deadlines, and both carry separate penalty charges if missed.

How Do Penalties Interact with Companies House Filing?

Your CT600 (corporation tax return) is filed with HMRC. Your annual accounts are filed with Companies House. These are separate obligations with separate deadlines and separate penalty regimes.

Companies House requires accounts within 9 months of the accounting period end (for private companies). Late filing penalties range from £150 (up to 1 month late) to £1,500 (over 6 months late).

HMRC requires the CT600 within 12 months. The penalties are as described above.

You can be penalised by both organisations simultaneously if you are late with both filings. The penalties do not overlap or offset. They are independent charges from independent bodies.

CT600 Penalties Compared to Self Assessment Penalties

Many company directors also file a personal Self Assessment tax return, and the two penalty regimes are easy to confuse. They are not the same.

Two differences stand out. First, Self Assessment has daily penalties — £10 a day for up to 90 days once you pass three months late — which corporation tax does not. Second, corporation tax tax-geared penalties are 10%, double the 5% used for Self Assessment, and they double again for repeat offenders. In other words, the corporation tax regime hits harder on the percentage charges, while Self Assessment hits harder in the three-to-six-month window through daily charges.

If you are a director, you are personally exposed to both. A bad year where you miss your CT600 and your personal return means two separate penalty trails from two separate parts of HMRC. The deadlines differ too: Self Assessment is due by 31 January following the end of the tax year, while your CT600 is due 12 months after your company's accounting period ends. Keep them on the same calendar so neither slips.

Corporation Tax Payment Methods and Processing Times

When paying late, the method you choose affects when HMRC considers the payment received:

If you are paying close to or after the deadline, use Faster Payments or CHAPS. Never rely on a posted cheque arriving on time.

HMRC's payment reference is your company's UTR followed by the accounting period end date. Getting the reference wrong can delay processing.

Can You Set Up a Time to Pay Arrangement?

If you cannot pay your corporation tax in full, you can contact HMRC to negotiate a Time to Pay (TTP) arrangement. This allows you to spread the payment over an agreed period, typically up to 12 months.

Key points about TTP:

• You must contact HMRC before the payment deadline if possible, or as soon as you realise you cannot pay.
• Interest continues to accrue during the payment plan. It is not waived.
• HMRC will ask about your company's financial position, including bank balances, expected income, and other liabilities.
• A TTP arrangement does not affect filing penalties. You must still file your CT600 on time.
• HMRC can withdraw the arrangement if you miss an instalment.

The phone number for HMRC's payment support is 0300 200 3835. Call early. If you wait until after enforcement action begins, HMRC is less likely to agree.

Penalty and Interest Changes Around April 2026

HMRC has been steadily raising the cost of non-compliance. Two shifts matter for any company with an accounting period ending in or after April 2026.

Higher interest. The late payment interest margin already rose from base rate plus 2.5 points to base rate plus 4 points in April 2025. With the Bank of England base rate near 4.5%, that keeps the effective late payment rate around 8.5% — historically high, and a real cost on any unpaid corporation tax. Repayment interest, by contrast, sits far lower at roughly 3.5%.

Tougher penalty enforcement. HMRC is applying its automatic penalty systems more aggressively and is quicker to escalate to debt collection. The practical effect for directors is that the "soft" period some companies used to rely on — where a late return drifted for months before HMRC chased it — has largely gone. Penalties are issued on schedule and pursued sooner.

If your year-end falls after April 2026, do not assume the figures you remember from a few years ago still hold. We track the detail in corporation tax penalties doubling in April 2026. The safest response to a rising-cost environment is simple: file and pay on time, every time, so none of the penalty or interest rates ever apply to you.

How AccountsOS Helps You Avoid CT600 Penalties

AccountsOS is built to stop you from ever being in this situation. Here is what the platform does:

Automatic Deadline Tracking

When you add your company, AccountsOS pulls your accounting reference date and calculates every key deadline: corporation tax payment, CT600 filing, confirmation statement, annual accounts. These appear on your dashboard and in Finn's proactive alerts.

Email and In-App Reminders

You receive reminders well ahead of each deadline, not just one notification on the day. Early warnings give you time to prepare your accounts and arrange payment.

AI-Powered Tax Estimation

Finn can estimate your corporation tax liability based on your transactions, so you know roughly what you will owe before the payment deadline arrives. No more guessing or scrambling at the last minute.

Companies House Sync

AccountsOS syncs directly with Companies House to ensure your deadline dates are accurate. If your accounting reference date changes, your deadlines update automatically.

For a complete walkthrough of how to file your corporation tax return, see our CT600 filing guide.

How to Prevent Late Filing in Future Years

If you have been caught out once, here are practical steps to make sure it does not happen again:

1. Set calendar reminders. Add your payment deadline (9 months and 1 day) and filing deadline (12 months) to your calendar as soon as your accounting period ends. Set reminders 3 months, 1 month, and 1 week before each date.

2. Start early. Do not wait until month 11 to think about your return. Begin gathering your records and speaking to your accountant (if you use one) at the 6-month mark.

3. Estimate and pay early. You can pay your corporation tax as soon as you know (or can estimate) the amount. Paying early means you earn repayment interest if you overpay, and you eliminate the risk of late payment interest.

4. Use accounting software. Tools like AccountsOS keep your transactions categorised throughout the year, so preparing your CT600 is straightforward rather than a last-minute scramble through bank statements and receipts.

5. Do not rely on your accountant alone. You are legally responsible. If your accountant is slow, chase them. If they consistently miss deadlines, find a new one. Better yet, use AccountsOS alongside your accountant to maintain visibility over your own deadlines.

6. Check your HMRC online account. Log in to your HMRC business tax account periodically to confirm your filing status and check for any outstanding penalties.

Corporation Tax Rates for 2026/27

For context, the current corporation tax rates are:

These thresholds are divided by the number of associated companies. If you control two companies, the small profits threshold drops to £25,000 each.

For a detailed breakdown including marginal relief calculations, see our corporation tax rates and thresholds guide.

What About Dormant Companies?

A dormant company is one that has had no significant accounting transactions during the period. Even dormant companies must file a CT600, unless they have formally told HMRC the company is dormant.

To notify HMRC that your company is dormant:

1. Write to your local Corporation Tax office or call HMRC.
2. Confirm that the company has not traded and has had no income or expenses.
3. HMRC will mark the company as dormant and will not issue a "notice to deliver" a CT600.

If you have not notified HMRC and they issue a notice to deliver, you must file. Failure to do so triggers the standard penalty regime, even though the company owes no tax.

Note that Companies House dormancy is separate from HMRC dormancy. You may still need to file dormant accounts with Companies House even if HMRC considers the company dormant.

Worked Example: Dormant Company That Forgot to File

This scenario is more common than most directors expect.

Company: Gamma Holdings Ltd (dormant, no trading activity) Accounting period ends: 31 December 2025 Corporation tax due: £0 Filing deadline: 31 December 2026 HMRC not notified of dormancy Actual filing date: 15 July 2027 (6 months and 15 days late)

Penalty Calculation

There is no interest because no tax was owed. But the director pays £500 purely for failing to file a return showing zero liability. Had they notified HMRC the company was dormant, no return would have been required and no penalty would have arisen.

This is one of the most frustrating penalty scenarios because it is entirely preventable with a single phone call or letter to HMRC.

What Records Do You Need to File Your CT600?

Gathering the right records before you start is critical. Missing documents are the most common reason directors delay filing, which pushes them past the deadline.

You will need:

• Bank statements for the full accounting period. Every account the company uses, including savings accounts and credit cards.
• Sales invoices for all income received during the period.
• Purchase invoices and receipts for all business expenses claimed.
• Payroll records if the company has employees (including yourself if you take a PAYE salary).
• Dividend vouchers for any dividends declared during the period.
• Loan agreements for any director's loans or third-party borrowing.
• Asset purchase records for any equipment, vehicles, or property bought or sold.
• Previous year's CT600 and tax computation for brought-forward figures.
• Companies House filings to confirm the accounting reference date and any changes.

If you use AccountsOS, most of this is already organised. Your transactions are categorised, receipts are stored against the relevant expenses, and your AI assistant can pull together the information your accountant (or you) needs to complete the return.

What If You Have Lost Records?

HMRC expects you to keep records for at least 6 years after the end of the relevant accounting period. If records are lost, you should reconstruct them as accurately as possible from bank statements, supplier records, and any other available sources. File the return with the best information you have and include a note explaining any estimated figures.

Filing with estimated figures is far better than not filing at all. An estimated return stops the penalty clock. You can amend it later if more accurate figures become available (within 12 months of the filing deadline).

Key Dates Summary: CT600 Filing and Payment

Here is a quick reference table using a 31 March 2026 year end as an example:

Frequently Asked Questions

What is the penalty for filing a CT600 one day late?

HMRC charges an automatic £100 penalty the moment your CT600 is one day past the 12-month filing deadline. There is no grace period and no warning. If your company has filed late in three consecutive periods, this penalty increases to £500.

Can I avoid CT600 penalties if my company has no tax to pay?

No. CT600 filing penalties apply regardless of your tax liability. A company with zero tax to pay will still face £100 at one day late, £200 at three months, and at least £800 after 12 months (due to the £300 minimum on the 6-month and 12-month penalties).

How much interest does HMRC charge on late corporation tax payments?

HMRC charges late payment interest at 8.5% per annum (as of 2026). This is calculated daily on the outstanding amount from the day after the payment deadline until the date the tax is paid in full. Interest applies to unpaid penalties as well as unpaid tax.

Will HMRC cancel my penalty if my accountant was late?

Almost certainly not. HMRC's consistent position is that the company director is responsible for ensuring the CT600 is filed on time. Your accountant acting as your agent does not transfer that legal obligation. You would need to demonstrate a truly exceptional circumstance beyond simply "my accountant did not file."

How long do I have to appeal a CT600 penalty?

You have 30 days from the date on the penalty notice to appeal. Late appeals are possible but you must explain why you did not appeal within the original window. Appeals can be made online through your HMRC business tax account or in writing.

What happens if I file my CT600 late but pay my tax on time?

You will face the filing penalties (£100 at 1 day, £100 at 3 months, percentage-based penalties at 6 and 12 months) but you will not be charged late payment interest, because the tax was paid on time. The 6-month and 12-month penalties are based on tax unpaid at the filing deadline, so if you paid on time, the tax-based element may be zero (though the £300 minimum still applies).

Can HMRC wind up my company for unpaid corporation tax penalties?

Yes. HMRC is the largest petitioner for compulsory winding-up orders in the UK. While they typically pursue this route for larger debts, it is within their power. More commonly, they will seek a County Court Judgment first, which damages your company's credit rating and your ability to obtain finance.

Does AccountsOS help with CT600 filing?

AccountsOS tracks all your corporation tax deadlines automatically. When you connect your company, we pull your accounting reference date from Companies House and calculate your payment and filing deadlines. Finn, our AI assistant, sends proactive reminders and can estimate your tax liability based on your transactions. While AccountsOS does not submit the CT600 to HMRC directly (that requires iXBRL-tagged accounts via HMRC-recognised software), it ensures you are prepared and never caught off guard by a deadline.

What is an HMRC determination and can I challenge it?

If you never file your CT600, HMRC can issue a determination — its own estimate of the tax due. The determination figure itself cannot be appealed, and it is enforceable like a self-assessment. The only way to replace it is to file your actual return, generally within three years of the filing deadline. Because HMRC tends to estimate high, a determination usually means paying more tax — and more tax-geared penalties — than you would have owed had you simply filed.

Do CT600 penalties apply to each return if my first year needs two returns?

Yes. If your first accounting period runs longer than 12 months, it is split into two corporation tax periods — the first 12 months and the remainder — each needing its own CT600. Each return has its own filing deadline and its own penalty exposure. Miss both and you face two separate sets of penalties, so first-year directors should diary all the dates carefully.

Are CT600 penalties tax-deductible?

No. Late filing penalties are not an allowable expense and cannot be deducted when calculating your corporation tax. Late payment interest on overdue corporation tax is treated differently — it is generally an allowable non-trading loan relationship debit, so the interest (but never the penalties) reduces your taxable profit. The simplest way to avoid the question entirely is to file and pay on time.