Corporation Tax Late Filing Penalties Doubling from April 2026

The First Increase in Nearly Three Decades

Corporation tax late filing penalties have not changed since 1998. For almost 28 years, the initial fine for missing your CT600 deadline has been £100. From 6 April 2026, that figure doubles to £200.

This is one of several changes landing on the same date, and while it might not grab headlines like the dividend tax increase or employer NI at 15%, it matters to every limited company in the UK.

The New Penalty Structure

Here is how the penalty escalation works from April 2026, compared with the current regime:

Filing delay	Current penalty	New penalty (from April 2026)
1 day late	£100	£200
3 months late	Additional £100	Additional £200
6 months late	HMRC estimates tax + 10% surcharge	HMRC estimates tax + 10% surcharge
12 months late	Additional 10% surcharge on estimated tax	Additional 10% surcharge on estimated tax

The flat-rate penalties at one day and three months both double. The percentage-based surcharges at six and twelve months remain the same, but they apply to HMRC's own estimate of your tax liability — which is often higher than your actual bill.

How the penalties stack up

If you file three months late under the new rules, you will owe £400 in penalties (£200 + £200) before any tax is even calculated. Under the current system, that same delay costs £200.

At six months, HMRC issues a determination — their best guess at what you owe — and adds a 10% surcharge on top. If they estimate your corporation tax at £20,000, that is a £2,000 surcharge plus the £400 in flat-rate penalties already accumulated. At twelve months, another 10% is added.

The message is clear: file late, pay more.

Who Is Affected

Every UK limited company that files a corporation tax return. There are no exemptions based on size, turnover, or whether you owe any tax. Even if your company made a loss and owes nothing in CT, the filing penalty still applies if you miss the deadline.

This catches dormant companies too. If your company is registered but not trading, you still need to file a CT600 unless you have formally told HMRC that the company is dormant. Many directors of inactive companies discover this the hard way.

When Your Return Is Due

Your corporation tax return is due 12 months after the end of your accounting period. For a company with a 31 March year-end, that means:

Accounting period ends: 31 March 2027
CT600 filing deadline: 31 March 2028
Tax payment deadline: 1 January 2028 (9 months and 1 day after the period ends)

Note that the payment deadline is earlier than the filing deadline. You must pay your estimated corporation tax bill within 9 months and 1 day of the period end, even though you have 12 months to file the return. Late payment carries its own separate interest charges — currently at 7.75% per annum.

Why This Matters More Than the Numbers Suggest

A £200 penalty for one day late might seem manageable. But consider three things:

Repeated offences compound. If you file late for two or more consecutive periods, the initial penalty doubles again — from £200 to £400 under the new rules. For a company that has been late before, the first day of delay now costs four times what it did under the old regime.

Estimated assessments are punitive. At six months, HMRC does not wait for you to file. They estimate your tax and bill you accordingly. These estimates are notoriously high because HMRC errs on the side of overcharging. You then need to file the correct return to get the difference refunded, which can take months.

Interest runs from day one. The late payment interest rate is currently 7.75%, calculated daily from the payment due date. Penalties and interest are separate — you can be charged both simultaneously.

Common Reasons Companies File Late

Most late filings are not deliberate. The typical causes include:

Not knowing the deadline. New companies often assume the deadline aligns with the Companies House accounts deadline. It does not.
Waiting for final figures. Directors delay filing because they are waiting for bank statements, invoices, or their accountant to finalise numbers. HMRC does not accept this as reasonable excuse.
Company dormancy confusion. A company that stopped trading but was not formally notified to HMRC as dormant still needs to file.
Accounting period mismatches. If your company changed its year-end, the filing deadline shifts. Some directors miss this.
Accountant delays. Your accountant filing late on your behalf does not excuse the penalty. The company — and ultimately its directors — are responsible.

What to Do Now

1. Check your filing deadline. Log into your HMRC business tax account or check your last CT600 acknowledgment. If you are unsure of your accounting period end date, Companies House records will confirm it.

2. Set calendar reminders. Set them at 3 months, 1 month, and 2 weeks before the deadline. A filing that takes 30 minutes should never cost you £200.

3. File early if you can. There is no advantage to waiting. You can file your CT600 as soon as your accounting period ends. Filing early does not change your payment deadline — you still have 9 months and 1 day to pay.

4. If you are already behind, file immediately. Penalties increase at each threshold: 1 day, 3 months, 6 months, 12 months. Every day you wait beyond a threshold risks the next penalty tier. Filing a return even with estimated figures is better than not filing at all — you can amend it later.

5. Check dormant companies. If you have a company that is not trading, make sure HMRC knows it is dormant. Otherwise, you are accumulating penalties for returns you did not know you needed to file.

Key Dates

6 April 2026 — new penalty rates take effect for accounting periods ending on or after this date
Your specific deadline — 12 months after your accounting period end date
Payment deadline — 9 months and 1 day after your accounting period end date

Frequently Asked Questions

Do the new penalties apply to my current accounting period?

The new penalty rates apply to corporation tax returns for accounting periods ending on or after 6 April 2026. If your accounting period ends on 31 March 2026, the current (lower) penalty rates still apply. If it ends on 30 April 2026 or later, the new rates kick in.

What counts as a "reasonable excuse" for filing late?

HMRC accepts very few reasonable excuses. Serious illness, the death of a partner, fire or flood destroying records, and HMRC's own service failures may qualify. Being too busy, waiting for your accountant, or not knowing the deadline does not count. If you believe you have a reasonable excuse, you must appeal in writing.

Can I appeal a late filing penalty?

Yes. You can appeal within 30 days of receiving the penalty notice. You will need to explain your reasonable excuse and provide supporting evidence. If your appeal is rejected, you can request a review or take the case to the tax tribunal. However, the success rate for appeals without a genuine reasonable excuse is low.

Does this affect Companies House filing penalties too?

No. These changes are specific to HMRC corporation tax return penalties. Companies House has its own penalty regime for late filing of annual accounts, which operates on a separate schedule and has its own rates. However, Companies House is making its own changes in 2026, including fee increases.

AccountsOS tracks your corporation tax deadlines and sends reminders automatically, so you never miss a filing date. Set up deadline tracking in the HMRC Hub to stay ahead of the new penalty regime.