Forms covered
| Form | Description |
|---|---|
| CT600 | Main Company Tax Return (2026, version 3). Declares profits and Corporation Tax, and shows whether tax or a repayment is due |
| CT600A | Close company loans and arrangements to confer benefits on participators (2026, version 3). Calculates section 455 tax on outstanding loans |
| CT600B | Controlled foreign companies, foreign permanent establishment exemptions and hybrid mismatches (2022, version 3) |
| CT600C | Group and consortium relief (2018, version 3), to claim or surrender losses between companies |
| CT600D | Insurance (2015, version 3), for insurers with overseas life assurance business |
| CT600E | Charities and Community Amateur Sports Clubs (2026, version 3), to claim exemptions and repayments |
| CT600 (Short) 2008 | Withdrawn historic short form. Use the current CT600 instead |
Deadlines
- The return must be filed within 12 months of the end of the accounting period.
- Corporation Tax is normally payable 9 months and 1 day after the period end.
- Large companies pay by quarterly instalments, and very large companies pay earlier.
- A late return attracts an automatic penalty.
- Section 455 tax applies to loans outstanding nine months after period end, at 35.75% for loans made after 6 April 2026.
What you need
- Corporation Tax UTR, Companies House number and exact accounting period dates
- Government Gateway login and authentication code
- Approved statutory accounts and a tax computation, both in iXBRL format
- Records supporting turnover, expenses, capital allowances, losses, interest, property income, gains and dividends
- Number of associated companies, group details, tax already paid and any claims such as R&D or group relief
Filing process
- Company information: name (box 1), Companies House number (box 2), UTR (box 3) and company type (box 4).
- Accounting period: start and end dates (boxes 30 and 35), no longer than 12 months.
- Attachments: confirm accounts and computations are attached (boxes 80 to 90) and tick supplementary pages (boxes 95 to 144).
- Income: turnover (box 145) and trading profit (box 155).
FAQs
Frequently asked questions
Which form does a standard limited company file?
The main CT600, submitted online with iXBRL accounts and a tax computation, adding supplementary pages only where they apply.
When is Corporation Tax due?
Normally 9 months and 1 day after the end of the accounting period. The return is due within 12 months.
Can an amended return be filed?
Yes, generally within 12 months of the filing deadline.