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CT600 filing errors, explained

Last updated: 9 August 2026

Overview. When you submit a Corporation Tax return (CT600), HMRC checks it and sends back a result. If it’s not accepted, HMRC returns a numbered error. The codes can look cryptic, so here’s what the common ones mean and what to do. If you reached this page from a link on your dashboard, it will have jumped straight to your error below.

When we stop a filing before sending it

Not every problem comes back from HMRC. Before we transmit anything, we run your return through HMRC’s own published validation rules, the same rule set their gateway applies. If the return would be rejected, we stop and tell you, and nothing is sent.

This is deliberately better than finding out afterwards. A rejected submission has to be investigated and resent, and if it happens close to your deadline it costs you time you may not have. A return stopped before sending is just a return you can still fix.

What to do. The message names the figure that is wrong, so go back to the wizard, correct it and file again. Your documents are still generated and still in your account, so you can look at the CT600 and the tax computation to see the figure in context. You are not charged again for the resubmission, because nothing was filed.

If a filing is blocked and you cannot see what to change, contact support with your filing reference. Being stopped by these checks is unusual, and if it happens on figures you believe are right, we want to know.

3304: Your return is already filed for this period

This accounting period has already been filed with HMRC, so this submission was not applied. HMRC only accepts one return per period, so a second “new” return for the same period is always rejected as a duplicate.

What to do. If you didn’t mean to submit again, there’s nothing to do. Your return is on HMRC’s record. If you need to change the figures you’ve filed, submit an amendment instead of a new return. See How to amend a CT600.

1611: HMRC can’t find an original return to amend

You submitted this as an amendment, but HMRC could not find an original CT600 matching the details you sent, so there’s nothing to amend. This almost always comes down to one of two things:

  • You meant to amend a return filed elsewhere (e.g. HMRC online or other software). HMRC matches the original on the UTR and the exact accounting-period start and end dates. If either doesn’t match what HMRC holds, they can’t find it. Check both carefully, then see How to amend a CT600 filed elsewhere.
  • You actually want to file for the first time. If no original has ever been filed for this period, this should be a new filing, not an amendment. Start a new filing instead.

If the UTR and periods are correct and it still fails, contact the HMRC Online Services Helpdesk on 0300 200 3600.

1046: HMRC didn’t accept your sign-in details

HMRC rejected the Corporation Tax credentials, so your return was not filed. Nothing was submitted and you won’t be charged again to resubmit.

HMRC sends this same error for several different problems, and only one of them is a wrong password. Three things all have to be right, and they all have to belong to the same HMRC account:

  • Government Gateway user ID and password: the ones you use to sign in to HMRC online services. If you’ve lost them, recover them at gov.uk.
  • Unique Taxpayer Reference (UTR): the 10-digit number on HMRC letters (such as the “Notice to deliver a Company Tax Return”) and inside your HMRC online account. Not your personal UTR, and not the Companies House number.
  • An account enrolled and activated for Corporation Tax for that company: correct sign-in details on an account that has never had Corporation Tax added, or where the posted activation code was never entered, are refused with this same error.

What to do. Sign in at gov.uk and check that Corporation Tax is listed on that account, that it shows the UTR you are filing with, and that it is active rather than awaiting activation. Then resubmit.

If you are confident all three details are correct and it is still refused, work through HMRC rejected my sign-in details but they are correct. If you have not set the account up yet, start with setting up a Government Gateway account for Corporation Tax.

3001: General submission error

This is a generic wrapper HMRC returns alongside a more specific error. The specific code (such as 3304 or 1611 above) explains the real problem. If 3001 is the only error shown, contact support and we’ll help.