Can I amend my first CT600 and file the second return together?

Yes. Where the first CT600 has been filed but needs correcting and the second short-period return is still outstanding, software can send the first as an amendment and the second as a new return in one filing journey. Each return covers its own Corporation Tax period.

Which statutory accounts go with the two returns?

If the first filing included only 12 months of accounts for a longer period of account, prepare the correct full-period statutory accounts. Both CT600s can include those same full accounts, with a separate computation for each tax period. HMRC describes this attachment arrangement in its Corporation Tax computations format guidance.

The first amendment is a regenerated return. Its tax figures can change when prepared from the full-period figures and allocations. Review the whole first CT600 as well as its accounts attachment.

If the full accounts already accompanied the first return and nothing needs correcting, consider filing only the remaining CT600. That separate option asks for short-period figures and handles accounts already supplied.

Example: accounts longer than 12 months

A company trading throughout 15 July 2024 to 31 July 2025 normally has a first CT600 for 15 July 2024 to 14 July 2025 and a second for 15 to 31 July 2025. If only the first is filed, the first is amended and the second is new. Both receive accounts for 15 July 2024 to 31 July 2025.

Use this route when the displayed CT600 dates match the first return you filed. If your Corporation Tax period differs from the accounts period, use the applicable different-period route. A dormant company should follow its HMRC notice rather than assume it needs two returns merely because its accounts are long.

What if the amendment deadline has passed?

You normally have 12 months after the statutory filing deadline to amend a return. See HMRC guidance on amendments and the filing-date rules. We do not calculate a deadline that blocks submission. HMRC determines whether it can accept and process the amendment. A gateway receipt alone does not establish that a late amendment has been processed.

HMRC handles each CT600 separately. Check the result for each period. One accepted return does not mean both were accepted. If a return is rejected, read its HMRC response before deciding what to file next. This route does not automatically retry just the rejected return.

How do I amend the first CT600 and file the second?

For a supported micro company with a full accounts period over 12 and no more than 18 months, follow the step-by-step filing guide. Use Amend filing made elsewhere even if you filed the first CT600 through this service. Select HMRC CT600 and then First CT600 already filed, second not yet filed under Filing purpose.

This choice is HMRC only. Selecting Companies House clears it. If you then file both CT600s to HMRC, both will be amendments.