Submitting a Corporation Tax return to HMRC requires a CT600 form and iXBRL-tagged accounts. BrightTax handles iXBRL tagging automatically for accounts prepared within BrightTax or imported from BrightAP. This article covers the full submission process from final checks through to HMRC confirmation.

• All CT return schedules must be completed and saved.
• Year End Accounts must be linked or an iXBRL file attached — see Linking Year End Accounts to a Corporation Tax Return.
• Your HMRC Corporation Tax filing credentials must be configured in System Settings > Account Information > HMRC Filing Details.
• The Declaration section (CT600 xii) must be completed.

1. Navigate to CT600 xii in the Table of Contents.
2. Switch to Edit mode.
3. Complete the declaration section — confirm the return is correct and complete to the best of your knowledge.
4. Click Save.

1. Click Check and Calculate from the bottom toolbar.
2. BrightTax will validate the return and calculate the CT liability.
3. Review and resolve any errors or warnings before proceeding.
4. Once the calculation is clean, review the CT600 to confirm the liability, repayment, and payment details are as expected.

Before submission, BrightTax generates the iXBRL-tagged computation file that accompanies the CT600 to HMRC. This happens automatically when you proceed to submission — you do not need to generate it manually.

1. Click Submit to HMRC from the bottom toolbar.
2. The submission window opens. Review the summary details — accounting period, CT liability, and linked accounts file.
3. Click Submit to transmit the return to HMRC.
4. A confirmation message will appear with the HMRC submission reference number.
5. The submission is recorded in the Submission History tab.

If a CT return needs to be corrected after submission:

1. Make the required corrections to the return.
2. Re-run Check and Calculate.
3. Click the three-dot menu (⋯) on the bottom toolbar.
4. Select Submit Amended.
5. Confirm and click Submit.

The Submit to HMRC button is not available

• Confirm your HMRC Corporation Tax filing credentials are entered in System Settings.
• Check that the Declaration (CT600 xii) has been completed and saved.
• Ensure Check and Calculate has been run with no outstanding errors.

HMRC is rejecting the submission with error 4066

• Error 4066 relates to an incomplete Corporation Tax chargeable entry on the CT600. This usually means the Marginal Relief schedule is missing. Add the Marginal Relief schedule via Template > Supplementary Schedules, re-run Check and Calculate, and resubmit.
• Check that the associated companies figure is correctly entered on the CT600 if the company has associated companies.

Q: Does BrightTax handle iXBRL tagging automatically?

A: Yes. For accounts prepared in BrightTax or imported from BrightAP, iXBRL tagging is handled automatically. For third-party iXBRL files, the tagging is done by the third-party software — BrightTax attaches and submits the file as provided.

Q: How do I know the submission was successful?

A: A confirmation message is shown immediately after submission and the submission appears in the Submission History tab with a status of Submitted and an HMRC reference number.

Q: Can I submit a CT return without attaching accounts?

A: In limited circumstances, yes — use CT Options under the Template tab to bypass the accounts validation and state your reason. However, HMRC expects accounts to accompany all CT returns.