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Authentication for CT600 fails repeatedly

Posted by James McCarthy 11 Oct 2026 at 20:10
First time user and CT600 keeps failing authentication.
I have triple checked the obvious things like UTR, User ID, PW and also logged in to Gateway separately to verify CT is enabled which it is. Having followed troupe shooting guide not sure what else to check ?

1 Comment

T
Tiny (AI Assistant) Staff 12 Oct 2026 at 09:21
Error 1046 is HMRC's generic "we couldn't authenticate you" response. It never says which part failed, and the return doesn't reach HMRC when it fires, so nothing has been filed twice and your figures are still saved. Since you've already ruled out the obvious things, these are the checks that most often find the cause when the details look right:

1. Use the sign-in test HMRC's filing service actually relies on. Go to https://www.tax.service.gov.uk/business-account and choose "Sign in with Government Gateway", not the GOV.UK One Login option. Type exactly the User ID and password you entered in TinyTax. CT600 filing only accepts Government Gateway credentials, and the Gateway password is separate from a One Login password even when the two accounts are linked. If you normally sign in to HMRC with an email address, that's One Login, and it won't work here.

2. Check that it's this company's Corporation Tax on that account. Once signed in with those same credentials, open Corporation Tax and compare the 10-digit UTR shown there digit for digit with the UTR on your return. The company UTR and a personal Self Assessment UTR look identical in format, and this is the most common cause of a 1046 that survives every other check. If you look after more than one company, make sure the Corporation Tax entry you're seeing belongs to the company you're filing for.

3. Give it time before trying again. Several failed attempts close together can make HMRC lock the credentials temporarily, and while that lasts every attempt returns 1046 even once everything is correct. Leave it at least an hour, ideally overnight, before the next try.

4. Type the password by hand when you retry. Open the period's "Review Error ▾" button on the dashboard, choose Review & Submit, then click "Update Credentials & Retry". Clear the password box completely and type the password in yourself so the browser or a password manager can't fill in an older one.

If the sign-in test in step 1 works and Corporation Tax for the right UTR is listed, but the submission still fails after waiting, HMRC's Online Services Helpdesk (0300 200 3600) can confirm whether that Gateway account is set up for Corporation Tax online filing for the company's UTR.

The full guide is here: https://tinytax.co.uk/support/hmrc-error-1046

If you'd like us to look at the failed attempts on your own account, email us from your registered address. We don't discuss account details on the public forum.