Troubleshooting HMRC Submission Errors | TinyTax Support

Troubleshooting HMRC Submission Errors

If you see an error when submitting your CT600 through TinyTax, it usually means HMRC has rejected something about your submission — not that TinyTax failed to connect. This guide covers the most common errors and how to fix them.

"Authentication Failure" (Error 1046)

This is the most common submission error. It means your Government Gateway credentials were rejected by HMRC.

Common causes:

  1. Wrong Government Gateway User ID or password — double-check your 12-digit User ID and password are correct.
  1. Using a personal login instead of your company's — your company has its own Government Gateway account for Corporation Tax. This is separate from any personal Self Assessment login you may have. Make sure you are using the credentials linked to your company's Corporation Tax service.
  1. Password needs resetting — if you have not used your Government Gateway credentials recently, HMRC may require a password reset. Visit the Government Gateway login page to reset it.
  1. Not enrolled for Corporation Tax online — if this is your first online filing, you may need to enrol your company for the Corporation Tax service through your Government Gateway account. Visit gov.uk/log-in-register-hmrc-online-services to check.
ℹ Your Government Gateway User ID is a 12-digit number (e.g., 123456789012). Your Company UTR is a separate 10-digit number. Make sure you are entering each in the correct field.

To fix this: Check your credentials, then click Retry in the submission popup. You do not need to re-enter your company figures — just update the credentials and submit again.

"UTR Not Found" or "Unable to Lookup Taxpayer"

HMRC could not match the Company UTR you entered to a registered company.

Common causes:

  1. Incorrect UTR — verify your 10-digit Company UTR. This is your company's Corporation Tax reference, not your personal Self Assessment UTR.
  1. UTR not linked to your Gateway account — the Company UTR must be registered to the same Government Gateway account you are using to submit. If you recently changed Gateway accounts, you may need to re-link your company.
Where to find your Company UTR: Check HMRC letters sent to your company, or log in to your Business Tax Account at tax.service.gov.uk.

"Accounting Period Does Not Match"

HMRC does not recognise the filing period dates you have entered.

Common causes:

  1. Period dates differ from HMRC's records — HMRC may have different period dates registered for your company. Check your Corporation Tax records in your Business Tax Account.
  1. Overlapping with a previous return — if you have already filed for part of this period, the dates may overlap. Contact HMRC to confirm the correct dates.

What "Fails to Connect" Actually Means

If you see a connection error, it does not mean TinyTax could not reach HMRC. In almost all cases, TinyTax successfully connected to HMRC and received an error response back. The error message from HMRC is shown in the submission popup.

If you are unsure what the error means, contact us at hello@tinytax.co.uk and we can check the submission logs for your company.

Still Stuck?

If you have checked your credentials and UTR but the error persists:

  • Call HMRC's Corporation Tax helpline: 0300 200 3410 (Monday to Friday, 8am to 6pm)
  • Contact us: Email hello@tinytax.co.uk with your company number and we will check the submission logs from our side

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