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Can't submit my Company tax return

Posted by Christopher Plummer 6 Jul 2026 at 16:30
I'm trying to submit my company tax return but your system is saying that my login details don't match Gov.uk. Or that I don't have a Gateway ID which I do and the corporation tax is showing on my account.
I'm trying to fix but not getting anywhere. Any thoughts?

1 Comment

T
Tiny (AI Assistant) Staff 6 Jul 2026 at 16:49
I can see what's happening here — your submissions have been hitting HMRC Error 1046 (Authentication Failure). Since Corporation Tax is already showing on your HMRC account, you are definitely enrolled, which is the main thing. The issue is which Gateway account the CT enrolment lives on.

Looking at the error logs, two different Government Gateway User IDs have been tried across your attempts. Only one of those Gateway accounts will have Corporation Tax enrolled for your company — entering credentials from a different account will always produce Error 1046, even if the password for that account is correct.

Error 1046 comes down to one of three causes:

Cause 1 — Wrong Gateway account. If you have more than one Government Gateway account (personal, business, or one a previous filer used), CT is enrolled on only one of them. You need that specific account's User ID and password.

Cause 2 — Temporary lockout. After multiple failed attempts, HMRC temporarily blocks further logins for an hour or two. Even with the correct credentials, retrying too quickly will keep returning 1046.

Cause 3 — GOV.UK One Login vs Government Gateway. If you recently started using GOV.UK One Login to access HMRC services, note that CT600 filing still requires the older Government Gateway credentials (the 12-character User ID), not your One Login email.

What to try now:

1. Go to gov.uk/log-in-register-hmrc-online-services and sign in — use whichever account shows Corporation Tax listed for your company
2. Note the exact 12-character Government Gateway User ID you signed in with (this is not your email address or a name — it is the alphanumeric code HMRC issued when the Gateway account was set up)
3. Wait at least an hour from your last attempt to let any temporary lockout clear
4. In TinyTax, when the submission credentials dialog appears, clear any pre-filled values and type that User ID and password manually
5. Reset the Gateway password first if you are not 100% certain it is current — go to "Problems signing in" on the Gateway page

For full guidance on all the causes of Error 1046, see: https://tinytax.co.uk/support/hmrc-error-1046

If you have tried all of this and it still fails, reply here with the rough time of your next attempt and we will check the logs again.