How to fix HMRC error 1046 (Authentication Failure) when filing a CT600

By DormantFile · Updated 22 July 2026

If HMRC rejected your CT600 with error 1046 — "Authentication Failure. The supplied user credentials failed validation for the requested service" — the return itself is fine. HMRC never even looked at it. The rejection means your Government Gateway sign-in failed for the Corporation Tax online service, and that distinction is the key to fixing it.

What error 1046 actually means

Error 1046 is raised by the Government Gateway, not by the Corporation Tax system. It fires when any one of these is true:

The User ID or password is wrong. The obvious case — but in practice the least common one for repeat failures.
The account isn't enrolled for the Corporation Tax online service. A Gateway account can sign in perfectly well on gov.uk and still have no CT enrolment. Filing software authenticates against the service, not just the account — no enrolment, instant 1046.
The enrolment was never activated. Enrolling triggers a letter with an activation code. Until that code is entered, the enrolment exists but doesn't work — and the code expires after 28 days.
The enrolment is for a different UTR. Each CT enrolment is tied to one company's 10-digit UTR. Sign in with an account enrolled for a different company and you'll get 1046 for this one.

A wrong password rarely fails twice in a row. If you've retried carefully and still see 1046, the cause is almost certainly one of causes 2–4.

How to fix it, in order

1. Confirm the sign-in works at all

Go to HMRC online services and sign in with exactly the credentials you used to file. If the sign-in fails there too, recover the User ID or reset the password from that page, or call HMRC's online services helpdesk on 0300 200 3600.

2. Confirm it's the company's account

A common trap: signing in with your personal tax account. It works on gov.uk, so everything looks fine — but it has no Corporation Tax service, so filing fails. The company needs its own business tax account. If you've only ever had a personal one, create a business account and enrol it (next step).

3. Check Corporation Tax is listed — and enrol if it isn't

Once signed in, look at the services attached to the account. Corporation Tax must appear as an active service. If it doesn't:

Enrol for Corporation Tax from within the account. You'll need the company's UTR and registered office postcode.
HMRC posts an activation code to the registered office — allow around 10 days.
Sign back in and enter the code. The enrolment only works after this step, and the code expires after 28 days (request a new one if it's lapsed).

This is the single most common cause of persistent 1046 errors: everything looks set up, but the activation letter is sitting unopened — or was never entered.

4. Check the UTR

The enrolment is tied to one company. If you look after more than one company, confirm the account you're using is enrolled for this company's UTR. The UTR is on every HMRC letter to the company (including the original CT41G).

5. File again

Error 1046 says nothing about the return's figures — once the sign-in works and Corporation Tax shows as an active service for the right UTR, the same return will go through.

If you file through an agent

Agents authenticate with their own agent Government Gateway account, which needs an agent Corporation Tax enrolment and authorisation for the specific client. If a director's own credentials work but an agent's don't, the missing piece is usually the client authorisation.

Still stuck?

HMRC's online services helpdesk (0300 200 3600) can see the state of the account, its enrolments, and pending activation codes — which is exactly the information you can't see from the outside. Have the company's UTR and company number ready.

Once your Gateway access works, filing the return itself takes a few minutes — see how to file a nil CT600.