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Amend only the second CT600 of a long accounting period

Last updated: 7 October 2026

Short version. If HMRC has accepted the second CT600 of a long accounting period, correcting it requires an amendment. Start with the full accounts dates, choose File only the remaining CT600 and enter corrected figures for that period only. This sends one amended return and leaves the first return untouched.

Go to the steps and entry details

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This HMRC-only route supports accounts longer than 12 months and up to 18 months when the first CT600 included the full accounts. It also supports a dormant remaining return. It does not cover charity CT600E returns or filing accounts at Companies House.

1. Amend only the second return

  1. Start an amendment. On your company page select Amend filing made elsewhere. Use this route even if the second return was filed through this service as part of a full-period filing.
  2. Enter the full accounts dates. Enter Period start and Period end for the full accounts period, for example 1 April 2024 to 31 May 2025. Do not enter only the short second period here. Select Continue to wizard.
  3. Choose HMRC. Under What are you filing? choose HMRC CT600.
  4. Choose the remaining return. Under Filing purpose choose File only the remaining CT600. Check that the Remaining return dates match the second period in your HMRC account, then tick the confirmation that the first return included the full accounts.
  5. Enter corrected figures. In Step 2 enter the corrected income, expenses and tax figures for the remaining period only. Include the applicable brought-forward balances. Do not enter the full accounts-period totals.
  6. Review the amendment. On the review page complete Reason for amendment and keep HMRC ticked.
  7. Check the free preview. Select Preview your documents (free), then Generate draft preview →. Check that the CT600 shows the remaining dates and is an amended return, with no accounts attached again. An active return includes its computation.
  8. Submit to HMRC. Select Continue to submission, complete the declaration and select Submit to HMRC. Check the submission result for acceptance.

2. Check the result and the remaining dates

HMRC error 3304, Original return already received for this period, means HMRC already holds an original for those dates. Choosing First CT600 already filed, second not yet filed still sends the second as a new return, so it cannot correct an accepted second return.

The remaining dates come from splitting the full accounts period at 12 months. You cannot edit them independently in this route. If they differ from HMRC's second-period dates, check your HMRC account before proceeding. Use a filing method that supports the correct date pair. HMRC decides whether an amendment matches its record.

The preview should show one amended CT600 for the remaining period. Box 90 explains that the full accounts accompanied the other return. HMRC checks that the accounts were supplied and the filing obligation is met. For an active company, check the remaining-period computation too.

Read why the second return can be amended separately, with a figures example. If the second return has never been filed, follow the original second-return steps instead.

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