Corporation Tax

HMRC error 1046: why your CT600 is rejected, and the fix

The most common first-filing rejection there is — and it's almost never the return that's wrong.

5 min readUpdated 13 July 2026

The short version

1046 means
'Authentication Failure' — the credentials don't match a Gateway account enrolled for the Corporation Tax service.
Usual cause
Your Government Gateway account isn't enrolled/activated for CT. Filing accounts at Companies House doesn't do this.
The fix
Enrol for Corporation Tax, wait for the activation code by post (~10 working days), activate, then re-file.
If still failing
Also check: right Gateway ID, right 10-digit UTR, CT service (not just PAYE/VAT) enrolled.

What error 1046 is telling you

Error 1046 is HMRC's Government Gateway saying "Authentication Failure": the credentials your filing software sent don't match a Gateway account that is enrolled and activated for the Corporation Tax online service. The return itself is usually fine — the door just won't open.

Why it hits first filings especially

New directors reasonably assume that having incorporated the company, and perhaps filed something at Companies House, means HMRC knows who they are. It doesn't work like that. The Corporation Tax online service is a separate enrolment on your Gateway account, and until you add it, no software can file your CT600 — it authenticates, fails, and returns 1046.

Run through the usual causes in order:

  • Your Gateway account isn't enrolled for Corporation Tax at all. Most common by far on a first filing.
  • It's enrolled but not activated — you have the enrolment but never entered the activation code.
  • Wrong Gateway ID — you're signed into a personal account, or a different company's account.
  • Wrong UTR — a mistyped or transposed 10-digit Unique Taxpayer Reference.

The fix, step by step

  • Sign in to your Government Gateway account (the company's, not your personal one).
  • Add Corporation Tax to the account. You'll need the company UTR.
  • HMRC posts an activation code to your registered office — allow up to about 10 working days.
  • Enter the code to activate the service, then re-file the CT600.

The real deadline risk is the postman

The return takes minutes; the activation code takes days. That gap is the number-one reason first-time filers miss their deadline. Enrol for Corporation Tax the moment you know a CT600 is on the horizon, not the week it's due.

How WrenTax does it

WrenTax runs an HMRC pre-flight before you file, so an enrolment problem surfaces as a clear, human explanation rather than a bare four-digit code. If a submission is rejected, you see the actual reason and resubmit with everything already prefilled — no retyping the return to fix an authentication issue. General information, not tax advice.

Common questions

What does HMRC error 1046 mean?

It's an authentication failure: the credentials you sent don't match a Gateway account that's enrolled and activated for the Corporation Tax online service. For a first filing the usual cause is that you've never enrolled the account for CT — filing accounts at Companies House doesn't enrol you with HMRC. Less often it's the wrong Gateway ID, or the wrong UTR.

How do I fix error 1046?

Enrol your Gateway account for Corporation Tax. HMRC then posts an activation code to your registered office, which takes up to about 10 working days, and you activate the service with it. Until that's done, no software can file your CT600 — so start it early, well before the deadline.

How long does Corporation Tax enrolment take?

It can take about 10 working days for the activation code to arrive by post. That is the single biggest reason first-time filers miss their deadline — the code, not the return. Enrol the moment you know a CT600 is coming.

Keep reading

Your first CT600 (the long-year split)ReadCorporation Tax deadlinesReadFiling dormant accountsRead

This guide is general information about UK company filing, not tax or legal advice. Figures and deadlines are current for 2026; always check your own dates against Companies House and HMRC. Register data © Companies House.