How do I file a company tax return?
Answered 11 March 2026
Here is a summary of how to file a company tax return, based on the available HMRC guidance sources:
How to File a Company Tax Return
HMRC Guidance / Practice
1. Trigger: Notice to Deliver A company must file a company tax return when served with a notice to deliver (form CT603) by HMRC.
2. Deadline The notice specifies a time limit for filing — normally 12 months after the end of the accounting period (AP). Failure to meet this deadline renders the company liable to penalties.
3. What the Return Must Include The company tax return consists of three components:
- The completed CT600 return form (plus any relevant supplementary pages, e.g. CT600A, CT600L, etc.)
- Accounts in iXBRL format (or PDF in limited cases — see exceptions below)
- Tax computations in iXBRL format, clearly showing how return figures are derived from the accounts
4. How to File: Online Only For accounting periods ending after 31 March 2010, the return must be filed electronically via the Corporation Tax Online Service. Any CT liability must also be paid electronically.
5. Software Options You can file using:
- HMRC's own Corporation Tax online filing service — intended for small, unrepresented companies; supports CT600 and supplementary pages CT600A, CT600E, and CT600J. ⚠️ This service will close on 31 March 2026.
- Approved third-party commercial software — can deliver the CT600, all supplementary pages, accounts, computations, and supporting documents.
6. Exceptions to Online Filing The only exceptions to mandatory online filing are:
- Companies run entirely by practising members of a religious society whose beliefs are incompatible with electronic communications
- Insolvent companies within a formal administration or winding-up procedure (note: solvent companies in a members' voluntary liquidation are not exempt)
7. Special Cases
- Unincorporated charities, clubs and societies: may file accounts in iXBRL or PDF, but must still file online and submit computations in iXBRL.
- Smaller charities (combined income ≤ £6.5m): accounts may be accepted in PDF format.
- Group accounts do not need to be iXBRL-tagged.
⚠️ Important notice: HMRC's free Corporation Tax online filing service (CATO) closes on 31 March 2026. Companies currently using it should prepare to transition to approved third-party software.
Citation sources
An electronic filer may deliver a return using: proprietary third-party software provided by a commercial company that can deliver the CT600, any of the required supplementary pages, the accounts, computations and any supporting documents HMRC Corporation Tax online filing service. This application is intended for small, unrepresented companies and allows them to complete and submit a CT600 and supplementary pages CT600A, CT600E and CT600J. This service will close on 31 March 2026. Find out what
The following forms are relevant to the company's obligation to comply with the requirements of a notice to deliver a return. Form Description CT114 Covering letter, available on SEES, to be used by the HM Revenue & Customs office when sending back an unsatisfactory return. CT208 Combined payment and return reminder issued by COTAX 42 days before the filing date if no payment has been received and no return has been logged for that AP. CT205 Return reminder issued by COTAX 42 days before the
For Accounting Periods (APs) ending after 31 March 2010, a company served with a notice to deliver must deliver a company tax return through the Corporation Tax Online Service with appropriate attachments. The company must also pay any CT or CT-related liability electronically for any accounting period. The only exceptions to submitting returns electronically for accounting periods ending after 31 March 2010 is where HMRC has agreed that the company can be treated as exempt on religious grounds
companies that are run entirely by individuals who are practising members of a religious society or order whose beliefs are incompatible with the use of electronic communications are not required to file online insolvent companies within a formal administration or winding up procedure are excluded from the legal requirement to file online. However, solvent companies being wound up under a members' voluntary liquidation are not excluded from online filing unincorporated charities, clubs and socie