Corporation Tax and the CT600 return
A UK company files its Company Tax Return (CT600) with HMRC within 12 months of the end of its accounting period and usually pays the tax 9 months and 1 day after the period ends. Profits up to £50,000 are taxed at 19%, profits above £250,000 at 25%, with marginal relief in between.
Key facts
| Main rate | 25% |
|---|---|
| Small profits rate | 19% on profits up to £50,000 |
| Marginal relief | Between £50,000 and £250,000, fraction 3/200 |
| Limits | Divided by one plus the number of associated companies, and reduced in proportion for a period shorter than 12 months |
| Filing deadline | 12 months after the end of the accounting period |
| Payment deadline | 9 months and 1 day after the end of the accounting period, unless quarterly instalments apply |
| Late filing penalty | £200, rising to £400 after 3 months, for returns with a filing date on or after 1 April 2026 |
As at 25 October 2026. Rates for the financial years 2023 to 2027 (each starting on 1 April).
Rates and marginal relief
Between the two limits the profit is taxed at the main rate, less marginal relief: 3/200 × (upper limit − augmented profits) × taxable profits ÷ augmented profits. A company with taxable profits of £100,000, no associated companies and no exempt distributions pays £25,000 − £2,250 = £22,750 for a 12-month period.
Accounting periods
An accounting period for Corporation Tax is never longer than 12 months. When the accounts cover a longer period, for example a first period of 15 months, there are two accounting periods and two CT600 returns: the first 12 months and the rest. Both attach the same accounts with a separate tax computation.
What a return contains and how it is filed
A return is the CT600 form with any supplementary pages, the statutory accounts and the tax computation, the accounts and computation in iXBRL. HMRC closed its free online filing service on 31 March 2026, so returns are now filed with commercial software, by the company or its agent. HMRC does not intend to introduce Making Tax Digital for Corporation Tax.
Late filing penalties apply even when no tax is due. A return delivered 18 months or more after the end of the period also attracts a penalty of 10% of the unpaid tax, and 20% after 24 months.
Frequently asked questions
When is Corporation Tax due?
For most companies, 9 months and 1 day after the end of the accounting period. Large companies, with profits above £1.5 million (a limit shared between associated companies), usually pay in quarterly instalments.
When is the CT600 return due?
Twelve months after the end of the accounting period it covers. When the accounts cover more than 12 months, both returns are due 12 months after the end of the period of account, if that period is no longer than 18 months.
What is marginal relief?
A reduction of the 25% main rate for companies whose profits are between £50,000 and £250,000, so the effective rate rises gradually from 19%. The limits are shared between associated companies.
Can I still file a CT600 on HMRC's website?
No. HMRC's free Company Tax Return service closed on 31 March 2026. Returns are filed with commercial software; paper is allowed only with a reasonable excuse or for returns in Welsh.
Sources
- HMRC: rates and allowances, Corporation Tax
- GOV.UK: Corporation Tax marginal relief
- Corporation Tax Act 2010, section 18D
- Finance Act 1998, Schedule 18, paragraph 14
- GOV.UK: Company Tax Return penalties for late filing
- GOV.UK: pay your Corporation Tax bill
- GOV.UK: accounting periods for Corporation Tax
- HMRC: closure of the service to file your company accounts and tax return
- HMRC: transformation roadmap (21 July 2025)
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Join the waitlistGeneral information about UK company law and tax, not legal or tax advice. The directors remain responsible for the accounts and returns they approve and file.