Compliance

What is CT600?

CT600 is the Corporation Tax return form companies submit to HMRC, declaring profits and calculating tax owed.

Current Rate (2025/26)

N/A - it's a form, not a rate

Example

Your accountant prepares the CT600 showing £80k profit, calculates £20k tax due, and files it online with HMRC.

Key Dates

Due 12 months after accounting period ends (but tax payment due at 9 months)

How CT600 Works in Practice

The CT600 is the official Corporation Tax return that every UK limited company must file with HMRC for each accounting period. It is not simply a summary of your profits -- it is a detailed declaration that covers taxable income, allowable deductions, capital allowances claimed, losses brought forward, and the final Corporation Tax calculation. Even if your company made no profit or was dormant, you still need to file a CT600.

The form itself is broken into several sections. The core sections cover turnover, income, deductions, and the tax computation. Supplementary pages are required if your company has specific circumstances -- for example, CT600A for loans to participators (directors), CT600B for controlled foreign companies, CT600C for group and consortium relief, CT600E for charities, and CT600J for disclosure of avoidance schemes. Most small Ltd companies will only need the core return and possibly CT600A if the director has an overdrawn loan account.

Filing is done electronically through HMRC's online service or via commercial software. You must also attach your company accounts (in iXBRL format) and a tax computation. The tax computation is different from the accounts -- it starts with the accounting profit and makes adjustments for items that are not allowable for tax purposes (such as depreciation, entertaining, and fines) and adds back capital allowances.

The deadline for filing the CT600 is 12 months after the end of your accounting period. However, the actual Corporation Tax payment is due earlier -- 9 months and 1 day after your accounting period ends. This catches many directors out. If your year end is 31 March 2026, tax payment is due 1 January 2027, but the CT600 is not due until 31 March 2027. Late filing attracts automatic penalties starting at £100, rising to £500 after three months, and eventually calculated as a percentage of the unpaid tax.

Step by Step

Your company's accounting period ends and your accounts are prepared. From those accounts, a tax computation is built that adjusts the accounting profit for non-allowable expenses, adds capital allowances, applies any losses brought forward, and arrives at the taxable profit. The CT600 captures this computation along with supplementary information HMRC needs.

Once complete, the CT600, accounts in iXBRL format, and tax computation are filed electronically with HMRC. Most accounting software can generate the CT600 and file it directly. After filing, HMRC processes the return and may raise enquiries within 12 months if anything looks unusual.

The Corporation Tax itself must be paid separately -- filing the CT600 does not trigger automatic payment. You pay via HMRC's online service, direct debit, or bank transfer using your company's UTR (Unique Taxpayer Reference) and the accounting period end date as the payment reference.

Practical Tips

Set a calendar reminder for 8 months after your year end -- this gives you time to prepare accounts before the 9-month tax payment deadline
Keep a running list of disallowable expenses throughout the year (depreciation, entertaining, fines) so the tax computation is straightforward
Pay Corporation Tax early if possible -- HMRC pays credit interest on overpayments, and you avoid any risk of late payment penalties
Use accounting software that generates iXBRL-tagged accounts automatically to avoid the hassle of converting them manually

Common Mistakes to Avoid

Confusing the filing deadline (12 months) with the payment deadline (9 months and 1 day) -- your tax is due before your return
Failing to file a CT600 for dormant periods, which still attracts late-filing penalties
Not attaching accounts in iXBRL format, which causes HMRC to reject the filing
Forgetting to add back disallowable expenses like depreciation and client entertaining in the tax computation

Frequently Asked Questions

Do I need to file a CT600 if my company is dormant?

Yes. You must file a CT600 for every accounting period, even if the company is dormant and has no income. HMRC will issue penalties for late filing regardless of trading status. You can notify HMRC that the company is dormant to avoid future returns, but any outstanding periods still need filing.

Can I file a CT600 myself or do I need an accountant?

You can file it yourself using HMRC's online service or commercial software. However, the tax computation and iXBRL accounts can be complex. Most directors use an accountant or software like Xero, FreeAgent, or AccountsOS to ensure accuracy and avoid penalties.

What happens if I file my CT600 late?

HMRC charges automatic penalties: £100 if up to 3 months late, another £100 at 3 months, 10% of the unpaid tax at 6 months, and a further 10% at 12 months. Interest also runs on any unpaid Corporation Tax from the due date.

What is the difference between the CT600 and company accounts?

Company accounts are filed with Companies House and show your financial statements. The CT600 is filed with HMRC and calculates your Corporation Tax. They use the same underlying figures but serve different purposes. Both are required for every accounting period.

Can I amend a CT600 after filing?

Yes. You can amend a CT600 within 12 months of the filing deadline (so up to 24 months after the accounting period ends). After that window closes, you would need to contact HMRC to request a correction.

Source: HMRC Company Tax Return guide: https://www.gov.uk/company-tax-returns

Related Terms

Corporation Tax

Corporation Tax is the tax UK limited companies pay on their profits. It's like income tax, but for businesses instead of people.

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