Troubleshooting
Fixing common CT600 submission errors
Authentication failures, invalid UTR, period conflicts, and business-logic rejections explained in plain language.
If HMRC rejects authentication, confirm the company is enrolled for Corporation Tax Online and that the credentials match the UTR on the return.
Invalid UTR almost always means a typo or a UTR that does not belong to that company number. Re-check both fields before rebuilding the XML.
Business-logic failures usually mean the figures or period dates contradict HMRC’s records (overlapping periods, missing prior return, or schema rules for older periods). Adjust the period or mark an earlier period as filed elsewhere, then resubmit.