CT600
The CT600 is a company's corporation tax return. It is filed online with the accounts and a tax computation in iXBRL format, due twelve months after the accounting period ends — three months after the tax itself is payable.
Also known as: company tax return, corporation tax return
How it works
The return reports taxable profit, the tax due, and any claims such as capital allowances, losses or R&D relief. It must be filed online with two attachments: the statutory accounts and a corporation tax computation, both tagged in iXBRL. HMRC rejects filings without them.
The timetable trips up new directors because the two dates are different. Corporation tax is payable nine months and one day after the accounting period ends; the CT600 is not due until twelve months after it. You pay first and file later. Interest runs from the payment date, and filing penalties start at ££100 the day after the filing deadline.
Supplementary pages attach to specific claims, and some claims have their own earlier deadlines — notably R&D, where the additional information form must reach HMRC before the return itself. Amendments can normally be made within twelve months of the filing deadline; after that, an overpayment relief claim is needed instead.
A company that has not traded may still receive a notice to deliver a return. If it does, the return must be filed even where the answer is nil, unless HMRC agrees the company is dormant for corporation tax.
Who this affects
- Every company with a notice to deliver a return, trading or not
- First-year companies whose long period requires two returns
- Companies making R&D, loss or capital allowance claims with their own deadlines
- Directors who paid tax on time but missed the filing date three months later
Common mistakes
- Assuming the payment and filing deadlines are the same date
- Filing without iXBRL-tagged accounts and computations
- Missing the twelve-month window to amend a return
- Ignoring a notice to deliver for a dormant company
Frequently asked questions
When is the CT600 due?
Twelve months after the end of the accounting period. Corporation tax itself is due earlier, nine months and one day after the period end.
What has to be filed with it?
Statutory accounts and a corporation tax computation, both in iXBRL format, submitted online.
What is the penalty for filing late?
££100 immediately, another ££100 after three months, then 10% of unpaid tax at six months and a further 10% at twelve months.
Can I amend a CT600 after filing?
Yes, normally within twelve months of the filing deadline. After that you need to make an overpayment relief claim.
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Reviewed by Waqas Sagar ACA FCCA FMAAT · Last reviewed 13 September 2026 · Figures for 2026/27 · About our practice
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