The CT600 is the Company Tax Return form a company files with HMRC for each Corporation Tax accounting period. It is filed online with the company's accounts and tax computation, and is due 12 months after the period ends.
What is filed
A Company Tax Return is more than the form. It is made up of:
- the CT600 itself, a set of numbered boxes summarising income, reliefs and the tax due;
- the company’s accounts, tagged in iXBRL;
- the tax computation, also in iXBRL;
- any supplementary pages that apply, such as CT600A for loans to directors or CT600L for research and development.
A return cannot cover more than 12 months. A longer period of account is split into two accounting periods, each with its own CT600.
Supplementary pages
A supplementary page is only filed when it applies to the company. Tax Optimiser files these with the CT600:
- CT600A: loans to participators by close companies
- CT600B: controlled foreign companies and hybrid mismatches
- CT600C: group and consortium relief
- CT600D: insurance
- CT600E: charities and community amateur sports clubs
- CT600F: tonnage tax
- CT600H: cross-border royalties
- CT600I: ring fence trades
- CT600J: disclosure of tax avoidance schemes
- CT600K: restitution tax
- CT600L: research and development
- CT600M: Freeports and Investment Zones
- CT600N: Residential Property Developer Tax
- CT600P: creative industries
Deadline and penalties
The return is due 12 months after the end of the accounting period. The tax itself is due earlier, usually 9 months and 1 day after the period ends.
| How late | Penalty |
|---|---|
| 1 day | £200 |
| 3 months | Another £200 |
| 6 months | HMRC estimates the bill and adds 10% of the unpaid tax |
| 12 months | Another 10% of the unpaid tax |
The £200 penalties apply to returns with a filing deadline on or after 1 April 2026. For a return that was due before that date, each is £100. If a company’s return is late three times in a row, the £200 penalties rise to £1,000 each (the £100 penalties to £500 each).
A return can normally be amended up to 12 months after its filing deadline.
Read more: The CT600 box by box, Submitting your Corporation Tax return and Amending a CT600.
