Error Code	Message	What this means
1046	Authentication Failure: The supplied user credentials failed verification for the requested service.	HMRC doesn’t recognise the details you’re using to submit your returns.

• Check you can log into the Government Gateway and the CIS Monthly Return option is available. If you can’t log in or the CIS Monthly Return option isn’t available, contact HMRC
• Check you've entered the Employer's PAYE Reference and Accounts Office Reference correctly. Go to Settings then Business Settings. Select Construction Industry Scheme and ensure these details appear correct
• Check you've entered the correct user ID and password. If the information appears to be correct, try deleting and re-entering it. This is to ensure there aren't any extra spaces or incorrect characters, and then submit again
  
  NOTE:

  Customer credentials must be 12 characters or less. If your password is more than 12 characters, only type the first 12 characters and then retry.

• Check your account with HMRC to see if you're registered for online filling
• If you're filing on behalf of a client, check you're registered as their agent with HMRC
• If the issue still occurs, contact HMRC’s online services helpline on 0300 200 3600

Next steps

Submit your CIS Monthly Return (CIS 300)