Definition
The CT600 is the Company Tax Return a UK company files with HM Revenue and Customs (HMRC) for each accounting period. It reports the company’s taxable profits and Corporation Tax due, and is filed online with the accounts and tax computation in iXBRL format. The deadline is 12 months after the accounting period ends. A company that receives a notice to deliver a return must file even if it made a loss or owes nothing.
How it is filed now
HMRC’s free Company Accounts and Tax Online (CATO) service closed on 31 March 2026, so companies now file through commercial software or an agent. Paper returns are only allowed with a reasonable excuse or when filing in Welsh.
Filing is not paying
Corporation Tax is due 9 months and 1 day after the period ends, nearly three months before the CT600 deadline. Where a new company’s first accounts cover more than 12 months, there are two accounting periods and two CT600s, as explained in your first Corporation Tax return.
Late filing penalties
For returns due on or after 1 April 2026, HMRC charges £200 once the return is a day late and another £200 at three months. At six months it estimates the tax and adds 10% of the unpaid amount, with a further 10% at 12 months. Repeated late returns attract higher flat penalties.
If you would rather hand the return over, you can file your CT600 with SmartFiling for a fixed fee.
Related questions
- When is the CT600 due? 12 months after the end of the accounting period it covers.
- Does a dormant company need to file a Company Tax Return? Only if HMRC sends it a notice to deliver one.
Related terms
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Corporation Tax
Corporation Tax is the tax UK limited companies pay to HM Revenue and Customs (HMRC) on their taxable profits.
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Accounting period
An accounting period is the period covered by a company's Company Tax Return (CT600) and used to calculate its Corporation Tax.
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Corporation Tax computation
A Corporation Tax computation is the working that shows how a company gets from the profit in its accounts to its taxable total profits and Corporation Tax bill.
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iXBRL
iXBRL (inline eXtensible Business Reporting Language) is a digital format that adds machine-readable tags to each figure in a set of accounts while keeping the document…
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CATO
CATO (Company Accounts and Tax Online) was HMRC's free online service for filing a company's accounts and Company Tax Return (CT600) with HMRC.
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