CT600 rejected by HMRC with error 1046: how to fix it

In short: A rejected Company Tax Return (CT600) submission isn't filed, so unless HMRC already holds a return for the period, the 12-month deadline still applies (GOV.UK: Company Tax Returns). Error 1046 means HMRC refused the Government Gateway sign-in, not your figures: check the user ID (not your email address), the password, and that Corporation Tax is added to that account, then send the return again. Taxley prepares and files the CT600 for £44.50 (promotion price until 31 Dec 2026; £89.00 from 1 Jan 2027) with micro-entity accounts, shows HMRC's reason for a rejection on the filing page and lets you send a paid return again at no extra charge.

Take the 30-second check to see whether Taxley fits your company, or start your return.

Key facts (checked on 1 October 2026)

What does it mean when HMRC rejects your CT600?

It means HMRC's system refused that submission, so it hasn't been filed. Error 3304 means HMRC already holds an original return; check it before resubmitting. A rejected amendment leaves the existing return unchanged. Otherwise the 12-month deadline still applies, and a return filed one day late costs £200 (GOV.UK: Penalties for late filing). Fix the reason HMRC gives, then send it again.

HMRC answers an online submission with an acceptance, or with one or more error numbers and its own text. Errors about the sign-in and HMRC's systems come from its gateway, and HMRC lists them in its Transaction Engine Document Submission Protocol. Errors about the return itself come from the Corporation Tax service, and HMRC lists those in its Corporation Tax online service validation rules. A rejection about the return often arrives as error 3001, a general code, with the specific reasons listed after it as further numbered errors, so read the whole list. HMRC's guidance on its closed filing service says you can re-file a rejected return using commercial software or through an agent, or on paper to the Corporation Tax office where HMRC allows a paper return (GOV.UK: Closure of HMRC's filing service). First accounts longer than 12 months may require one or two returns, depending on trading dates and HMRC's notice (GOV.UK: first accounts and Company Tax Return). Check acceptance for every required return.

What does "Authentication Failure. The supplied user credentials failed validation for the requested service" mean?

It's HMRC's error 1046, and it means HMRC refused the Government Gateway sign-in sent with the return. HMRC's gateway checks the user ID and password, and that the account may file Corporation Tax for this company (HMRC: Transaction Engine Document Submission Protocol). It says nothing about your figures: HMRC stopped at the sign-in.

The protocol explains why one error covers several problems. For a service such as Corporation Tax, HMRC authenticates the user ID and password, and the tax record, identified by the company's reference, can be used only by a client enrolled in the service or an agent authorised for it. So 1046 appears when the user ID or password is wrong, and also when the details are right but belong to an account without this company's Corporation Tax, such as a director's personal Self Assessment sign-in. It appears when an email address is typed where the user ID belongs: GOV.UK lists two ways to sign in, Government Gateway with "a user ID (up to 12 characters) and password" and GOV.UK One Login with "an email address and password" (GOV.UK: HMRC online services). An agent's sign-in without the company's authorisation fails the same way, as does a sign-in for a company whose Corporation Tax was added but not yet activated with the code HMRC posts.

How do you fix HMRC error 1046?

Test the sign-in on GOV.UK before you send the return again. Sign in to the company's HMRC business tax account with the same user ID and password, and check that Corporation Tax for this company is listed there (GOV.UK: Add Corporation Tax services). Resubmit only once both checks pass.

1. Use the Government Gateway user ID, not an email address. The ID has up to 12 characters (GOV.UK). If a browser or password manager filled the box in, check what it entered.
2. Use the account that holds the company's Corporation Tax. HMRC's guide says the business tax account shows a tax card for each tax registered with that user ID, and a missing tax can mean you're signed in with a different ID (HMRC: business tax account guide).
3. Add and activate Corporation Tax if it isn't listed. HMRC posts an activation code to the registered office within 10 days, or 21 if you live abroad. The Corporation Tax activation code guide covers the steps.
4. Check the UTR on the return. It must be the company's 10-digit Unique Taxpayer Reference, not a director's personal one (GOV.UK: Find your UTR number).
5. Agents use their own sign-in. An agent must not use a client's sign-in details (GOV.UK: tax agent authorisation), so the practice needs the company's authorisation for Corporation Tax.
6. Stop after repeated failures. Too many wrong sign-in attempts lock a Government Gateway account, which unlocks after 2 hours (GOV.UK: problems signing in).

If you can't find the user ID at all, the guide to the Government Gateway user ID for Corporation Tax covers where to look and how to recover it. GOV.UK says you can recover or reset lost sign-in details, and that you'll be told how when you try to sign in (GOV.UK: problems signing in). Avoid creating a second user ID to get round the problem: HMRC's business tax account guide says to use only one Government Gateway user ID for the business and not to create a new one (HMRC: business tax account guide). A new ID starts without Corporation Tax, so you'd have to add it again and wait for another activation code by post. If the deadline is close, file as soon as the sign-in works on GOV.UK, and keep a note of each attempt and HMRC's error text in case you later need to show a reasonable excuse for a late filing penalty (GOV.UK: penalties for late filing).

Why do so many CT600 rejections come from the sign-in?

In Taxley's own live filings, the sign-in error, 1046, is the most common reason HMRC rejects a return, and a frequent cause is an email address typed where the Government Gateway user ID belongs. Directors now meet several sign-in methods, and only the Gateway user ID files a CT600 for HMRC (GOV.UK: HMRC online services).

HMRC's own filing service closed on 31 March 2026, and companies now file through commercial software (GOV.UK: Closure of HMRC's filing service). That software sends the user ID and password with the return, as HMRC's gateway requires, so a wrong entry only shows up when HMRC answers. Email-based sign-ins have also spread. Companies House WebFiling has needed GOV.UK One Login, which signs in with an email address, since 13 October 2025 (GOV.UK: Filing your Companies House information online). HMRC began offering One Login to new individual customers on 9 February 2026, while customers with a Government Gateway account keep using their Gateway ID, even if they have a One Login for Companies House (HMRC: One Login for new customers). In Taxley's experience, a browser filling in a saved email address is one way the wrong details reach HMRC. The fix is the same each time: find the Gateway user ID that holds the company's Corporation Tax and test it on GOV.UK first.

What do HMRC's other CT600 rejection codes mean?

Each code points to one cause. Sign-in and gateway errors come from HMRC's Transaction Engine protocol; errors about the return come from HMRC's Corporation Tax online service validation rules. Find the number in HMRC's message, then match it in the decision table below before you change anything.

Taxley's decision table: HMRC rejection codes, causes and fixes

HMRC's validation rules give the condition behind each return error. Error 1608 means the UTR in the return doesn't exist on HMRC's records. Error 7752 means the company registration number in the return doesn't match the one HMRC holds for that UTR, and 7782 means the accounts' period doesn't overlap the CT600's period by at least a day. Error 3320 means the tax computation uses a format, called a taxonomy, whose dates don't cover the period, which software has to fix. The amendment errors cover an amended return whose dates differ from HMRC's records (1610), one with no original to amend (1611) and one sent more than 12 months after the filing deadline when no enquiry is open (1612). Codes from 9100 to 9999 are business rules in HMRC's CT600 specification, such as a box that needs another box completed (HMRC: CT online validation rules). Error 1000 is HMRC's own "System failure", so wait and resend (HMRC: Transaction Engine protocol).

What should you do when HMRC says it already has a return for the period?

Stop and check before you send anything else. Error 3304, "Original return already received for this period", means HMRC already holds an original return for those dates and won't take a second one; a change must be sent as an amendment instead (HMRC: Corporation Tax online service validation rules).

Find out which return HMRC holds before deciding what to send. Ask anyone who might have filed for the company, such as a former accountant, and look for an earlier HMRC acknowledgment from your own software. In Taxley's experience, the usual causes are a return already filed elsewhere for those dates, or accounting period dates on HMRC's records that don't match the company's accounts. Check the dates in the company's business tax account, and ask HMRC through its Corporation Tax enquiries page if they aren't clear. If HMRC's dates are wrong, ask HMRC to correct them before you file. If HMRC holds a return with figures you need to change, amend that return, usually within 12 months of the filing deadline (GOV.UK: Company Tax Returns: making changes). Don't send your return as an amendment just to get past the error, because an amendment changes the return HMRC already holds. If the company was dormant, ask HMRC whether it still needs this return.

Is a rejected CT600 the same as a mistake in a filed return?

No. This submission was rejected, so it wasn't filed: correct the cause and send it again. Error 3304 means HMRC already holds an original return; check it before resubmitting. A rejected amendment leaves the existing return unchanged. A mistake in a return HMRC accepted needs an amended return, usually within 12 months of the filing deadline (GOV.UK: Company Tax Returns: making changes).

HMRC's checks look at the sign-in, the format and the consistency of the return, not at whether the figures are right. GOV.UK says the acknowledgment of an online return doesn't mean HMRC has agreed the figures, and that HMRC can correct obvious errors or enquire into the return (GOV.UK: Company Tax Return obligations). So a return can pass every check and still treat dividends as an expense or leave out a director's loan. The guide to common CT600 mistakes when you file it yourself covers those errors and how to spot them, and the guide to correcting a CT600 after filing covers amending a return HMRC has already accepted. If you're not sure whether a return was filed, rejected or is still awaiting HMRC's answer, read how to tell whether HMRC has received your CT600 before you send it again.

How does Taxley help when HMRC rejects a return?

Taxley (taxley.co.uk), UK online software that prepares and files the Company Tax Return (CT600) with HMRC and the annual accounts with Companies House, shows HMRC's reason for a rejection on the filing page and in an email, and lets you resend a paid return at no extra charge. It explains sign-in (1046) and duplicate-period (3304) errors in plain English.

The filing page lists each error number with HMRC's own text, a reference for the attempt and, while the return can be changed, a button to edit it. After a 1046, a panel says your return and payment are fine and that nothing has been filed yet, and lists the usual fixes: the account that holds this company's Corporation Tax, a user ID of up to 12 letters and numbers rather than an email address, and the exact password. Before anything reaches HMRC, Taxley removes spaces from the user ID and refuses one that looks like an email address. For a 3304, it tells you not to resubmit and to check the period dates with HMRC. Taxley never saves the password to your account or filing. It is held encrypted while sending; if sending fails, the encrypted record is deleted within three days. A "Run a test" rehearsal uses Taxley's own test sign-in, so a 1046 there is Taxley's problem, not yours. Taxley checks each return against HMRC's CT600 schema, and several of HMRC's business rules, before it takes payment. Take the 30-second check to see whether Taxley fits your company.

Frequently asked questions

Does a rejected CT600 count as filed on time?

No. This submission was rejected, so it hasn't delivered the return, and the 12-month deadline still applies; a return filed one day late costs £200 (GOV.UK: Penalties for late filing). Send it again once the cause is fixed. The exception is error 3304: HMRC already holds an original return, so check it before resubmitting.

Can repeated 1046 errors lock my Government Gateway account?

Entering wrong sign-in details too many times locks a Government Gateway account, and GOV.UK says it unlocks after 2 hours; helpdesk advisers can't unlock it (GOV.UK: problems signing in). Test the details by signing in on GOV.UK rather than resubmitting repeatedly.

Do I have to pay again to resubmit a rejected return?

That depends on your software's terms. Taxley charges once per return: if HMRC rejects it, you correct it and send it again at no extra charge. A sign-in error (1046) needs no change to the return at all, only working sign-in details.

Can my accountant's login fix error 1046?

Only if the accountant files the return as your agent. GOV.UK says you must not give your sign-in details to an agent, and an agent must not use a client's (GOV.UK: tax agent authorisation). An agent files with the practice's own sign-in and your authorisation.

What does error 3001 mean on a CT600?

It's HMRC's general business-error code: "The submission of this document has failed due to departmental specific business logic in the Body element" (HMRC: Transaction Engine protocol). The specific reasons follow it as further numbered errors, so fix those.

General information, not personalised tax advice. HMRC error codes and wording checked against HMRC's published specifications on 1 October 2026.