NCTS6 Error Messages: HMRC Advice on Third Party Software Error Messages

BY:

Gail Leeson
2 June 2026

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HMRC have advised on the action to be taken when users experience declaration error messages after the NCTS6 update on 1st June.

Users have reported receiving third-party software error messages in which declarations return HTTP 500 (“bad error”) responses.


The issue is currently under urgent investigation by HMRC, which is working directly with affected software developers.


Action to be taken

If you are a user of third-party software and receive this error response when submitting a declaration, the prescribed action is as follows:

  • Review all declaration data fields to ensure the correct information has been entered
  • Amend any data that has been entered incorrectly
  • Attempt to resubmit your declaration


HMRC requests that the error not be escalated through the NCTS Helpdesk or Supervising Office, as they have sufficient working examples to fully investigate.


A further update is expected in due course.


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