CT600 rejection workflow

Government Gateway CT600 rejection troubleshooting.

A practical guide for practices handling rejected Corporation Tax submissions, validation errors, iXBRL attachment issues, authorisation problems, and evidence-preserving resubmission.

Do not treat a rejected CT600 as filed

A rejected CT600 is not the same as a filed return. The practice should preserve the HMRC response, identify the cause, correct the submission package, reapprove if the change is material, and resubmit before relying on the return as complete.

The practical aim is to fix the problem without losing the evidence trail. A clean workflow explains what was sent, what HMRC returned, what changed, who approved the change, and what was successfully accepted afterwards.

First triage: classify the rejection

Most CT600 rejection work falls into a few categories. Classifying the issue early stops the team from editing the wrong part of the return.

  • Authentication or Government Gateway authorisation problems.
  • Company identity, UTR, or accounting-period mismatch.
  • CT600 validation rules or schema/version issues.
  • Accounts or computations attachment problems.
  • iXBRL taxonomy, tagging, encoding, or file-format problems.
  • Supplementary-page indicator and detail mismatches.
  • HMRC service availability or delayed-response incidents.

Authorisation and identity checks

Before changing tax figures, confirm the basics. Is the correct company UTR in use? Is the accounting period the one HMRC expects? Is the agent authorised for Corporation Tax? Are the Government Gateway credentials and permissions correct for the company or agent service being used?

These checks sound obvious, but they are common causes of wasted time. If the submission package is correct but the account cannot file for that company, editing the CT600 will not fix the rejection.

Validation and schema failures

HMRC publishes Corporation Tax online service validation rules and technical guidance. These rules cover CT600 schema versions, data formats, box consistency, attachments, iXBRL requirements, and other filing constraints.

A good troubleshooting process keeps the raw error text or code with the client file, maps it to the likely validation area, and records the correction. That avoids a weak audit trail where the team only knows that "the return was rejected and then fixed".

iXBRL accounts and computations issues

Many CT600 filings rely on iXBRL accounts and computations. HMRC guidance explains that accounts and computations must be prepared in the required online filing format, and technical guidance includes iXBRL expectations such as taxonomy and document requirements.

If the rejection points to attachments, check that the files are present, readable, for the correct accounting period, tagged appropriately, encoded correctly, and attached in the expected part of the submission package.

Supplementary-page mismatches

CT600 supplementary pages can create rejections or review failures when the main return indicator says one thing and the attached detail says another. Examples include CT600A loans to participators, CT600C group relief, CT600L R&D, CT600N RPDT, or CT600P creative industries.

Before resubmission, check the indicator, the supplementary page, the computation, and the supporting evidence together. Changing one page may affect the final tax payable, repayment, payable-credit, or client approval position.

Evidence-preserving resubmission checklist

  • Save the original submission timestamp and HMRC response.
  • Classify the rejection and identify whether figures changed.
  • Correct the CT600, supplementary page, attachment, or authorisation issue.
  • Rebuild or revalidate iXBRL files where attachment issues are involved.
  • Ask for renewed client approval where the tax position or disclosures changed.
  • Resubmit and save the new HMRC acknowledgement or rejection response.
  • Reconcile payment, repayment, or interest consequences after acceptance.

How Robocount supports rejected-return handling

Robocount is built around structured CT600 preparation, review, filing, and evidence. Rejection handling belongs in that same workflow, because a failed submission is part of the filing history.

  • Keep CT600, supplementary pages, computations, and iXBRL attachments together.
  • Make submission evidence and HMRC responses part of the client file.
  • Support reviewer checks before a changed return is resent.
  • Keep post-filing payment and reconciliation work connected to the accepted liability.
  • Help practices explain what changed between rejected and accepted submissions.

FAQ

Can a practice ignore a rejected CT600 if the figures are correct?

No. If HMRC rejects the online submission, the return has not been accepted. The practice needs to fix the filing issue and obtain a successful response.

Should the client approve a rejected return again?

If the correction changes tax figures, disclosures, attachments, or filing position, renewed approval is sensible. Even where the fix is technical, keep a note explaining why approval was or was not refreshed.

Where should the HMRC rejection response be stored?

Store it with the CT600 evidence file, alongside the submitted return package, computations, attachments, approval record, and final accepted acknowledgement.

Useful HMRC references

This guide is general product and filing workflow information, not tax advice. Check HMRC's current service guidance and the actual rejection response before deciding what to change.