A 1046 error is an authentication failure that occurs when submitting a return to HMRC. It happens when a user's credentials are not recognised.

The most common causes are:

• Your Government Gateway user ID or password is wrong
• The Corporation Tax service has not been added or activated on your Government Gateway account
• An invalid UTR (Unique Taxpayer Reference), or a problem with the HMRC Gateway itself
• A previous accountant filed on your behalf using their own Agent Account, meaning the UTR may still be linked to their details

To resolve it, you should check that your Government Gateway credentials are correct (UTR, username and password), ensure the corporation tax service is activated in your Gateway account, and confirm your UTR matches what is shown in the Corporation Tax section of your Gateway account.