Fix HMRC Error 1046 (Authentication Failure) on CT600 Submission | TinyTax Support

Fix HMRC Error 1046 (Authentication Failure) on CT600 Submission

CT600 rejected with HMRC Error 1046 (Authentication Failure)? Learn the five causes — wrong credentials, Corporation Tax not enrolled, GOV.UK One Login, lockout, stale password — and how to fix each, including when HMRC confirms sign-on works but submission still fails.

If your CT600 submission is rejected with HMRC Error 1046 — "Authentication Failure", it means HMRC did not accept the credentials TinyTax sent on your behalf. The return never reaches HMRC's records, which is why nothing shows up on your HMRC online account afterwards. This guide explains the causes and how to work through them.

⚠ Don't keep retrying. HMRC may temporarily lock the account after a handful of rapid failures, and further attempts will keep returning 1046 even after you've fixed the underlying problem. Wait at least an hour between attempts while you diagnose.

What Error 1046 Actually Means

Error 1046 is HMRC's catch-all "we did not authenticate you" response. It does not tell you why, only that one of the following things is wrong:

  1. The Government Gateway User ID and password don't match an HMRC account
  2. The account exists but isn't enrolled for the Corporation Tax service for this company
  3. You're entering GOV.UK One Login credentials instead of the older Government Gateway credentials — only the Gateway login works for CT600 filing
  4. HMRC has temporarily locked the account because of repeated failed attempts
  5. A saved or autofilled password that's now out of date is being sent instead of your current one
You need to rule them out in order.

ℹ If you've phoned HMRC and they told you sign-on is "Successful Through Software" but the CT600 submission still returns 1046, that actually narrows it right down. "Sign-on successful" means your Government Gateway User ID and password are valid and did log in — so it is not a wrong-password problem, and resetting the password won't fix it. What's left is Cause 2 (the account is signed in but not enrolled and activated for the Corporation Tax service for this company's UTR) or a UTR that doesn't match HMRC's record (Cause 1). Work through those two. Because Error 1046 is HMRC's generic authentication response used by all filing software, an HMRC adviser reporting "several software users affected" is describing the enrolment/UTR pattern, not a fault in one product.

Cause 1: Wrong Government Gateway Account or UTR Mismatch

The Government Gateway User ID you entered must be the one that holds the Corporation Tax enrolment for this specific company. The 10-digit Company UTR must also be the one HMRC has on record for that company.

Check:

  1. Go to your Business Tax Account: https://www.tax.service.gov.uk/business-account
  2. Sign in with the same User ID and password you tried to use in TinyTax
  3. Confirm you can see "Corporation Tax" listed for this company
  4. Click Corporation Tax and read the 10-digit UTR shown on that overview page. Compare it digit for digit with the UTR on your TinyTax return
If sign-in fails, you have the wrong credentials. If sign-in succeeds but Corporation Tax isn't listed, skip to Cause 2. If the UTR doesn't match, correct it on your return and that alone will usually fix the error.

⚠ <strong>Check the UTR even if you're certain it's right.</strong> This is the single most common cause of a 1046 that survives every other fix. Your <strong>company</strong> UTR and your <strong>personal Self Assessment</strong> UTR are both 10 digits and both come from HMRC, so a personal UTR saved years ago and reused ever since looks completely correct — and HMRC's error message never tells you the number is the problem. Many company directors don't realise the company has a UTR of its own. The only reliable check is the one above
ℹ A Government Gateway User ID is usually 12 digits (e.g., 123456789012). Your Company UTR is a separate 10-digit number found on HMRC letters addressed to the company. The two are often confused — make sure each goes in the correct field.

Common mix-ups:

  • Entering your personal Self Assessment UTR instead of the company's Corporation Tax UTR
  • Using a personal Self Assessment Gateway login instead of the company's
  • Using your accountant's old Gateway login (they may have de-authorised the company)
  • Typing the UTR with spaces or hyphens — enter all 10 digits with no separators

Cause 2: Company Not Enrolled for Corporation Tax Online

Even if you can sign into Government Gateway, your company has to be specifically enrolled for the Corporation Tax service. New companies often have a Gateway account but no CT enrolment yet.

Check:

  1. Sign into your Business Tax Account
  2. Look for "Corporation Tax" in the list of services on the home screen
  3. If it's not there, you need to add it
To enrol:

  1. Visit gov.uk/log-in-register-hmrc-online-services
  2. Sign in with your Government Gateway User ID
  3. Click "Add a tax, duty or scheme"
  4. Select Corporation Tax
  5. Enter your 10-digit Company UTR
HMRC will post an activation code to the company's registered office. This typically takes 7 working days. You cannot submit a CT600 online until you receive the code and activate the service. There is no way to speed this up — it must be posted.

⚠ If you're close to your filing deadline and Corporation Tax isn't enrolled yet, request the activation code immediately. The post-and-activate cycle takes about 10 days end to end.

Cause 3: GOV.UK One Login vs Government Gateway

Since 2024 HMRC has been migrating sign-in to GOV.UK One Login — the unified email-and-password login that works across many government services. The HMRC website now lets you sign in either way, but the CT600 submission API still only accepts the older Government Gateway credentials. Until HMRC migrate the API (currently expected 2026–2027), TinyTax has to use the Gateway path.

Signs this is the cause:

  • You recently set up or were prompted to use GOV.UK One Login on HMRC's website
  • You can sign into your Business Tax Account fine, but the CT600 submission keeps failing with 1046
  • The User ID you've been entering looks like an email address (`[email protected]`) rather than a 12-digit number
What to do:

  1. Go to the Business Tax Account sign-in page: https://www.tax.service.gov.uk/business-account
  2. Choose "Sign in with Government Gateway" — the older option, not the One Login one
  3. Enter your 12-digit Gateway User ID (no email, no spaces) and your Gateway password (this is a separate password from your One Login password — even if the two accounts are linked behind the scenes, they don't share a password)
  4. If sign-in succeeds, those are the exact credentials TinyTax needs — re-enter them on your submission page and retry
  5. If you've forgotten your Gateway password, use "Problems signing in" → "I've forgotten my password" on that same screen to reset it. Resetting the One Login password does not reset the Gateway password.
⚠ GOV.UK One Login and Government Gateway are two different sign-in systems with two different passwords. The Gateway User ID is 12 digits, the One Login user ID is your email address. Make sure you're typing the 12-digit Gateway User ID and the Gateway password into TinyTax, not your One Login email and password.

If the Gateway sign-in itself fails (not just the CT600 submission), your account may need to be re-paired with the Gateway side. Call HMRC's Online Services Helpdesk on 0300 200 3600 and ask them to confirm Gateway sign-in is still active for the company's UTR.

Cause 4: HMRC Throttle / Temporary Lockout

If you've already retried several times in quick succession, HMRC may have temporarily locked the account. Once locked, every subsequent attempt will return Error 1046 even if your credentials are correct and CT is enrolled.

Signs this is the cause:

  • 3+ failed submissions in under 30 minutes
  • You've already verified your credentials work in the Business Tax Account
  • Corporation Tax shows as enrolled
To clear it:

  1. Wait at least 1–2 hours since your last attempt — preferably longer
  2. Reset your Government Gateway password as a precaution: Government Gateway login page → "I have forgotten my password"
  3. Sign into the Business Tax Account once with the new password to confirm it works
  4. Retry submission in TinyTax
A password reset often clears the lockout immediately. If retrying still bounces with 1046, leave it overnight and try once the next morning.

Cause 5: A Saved or Autofilled Password That's Now Out of Date

This one catches people who've filed successfully before and have just changed their Gateway password. There are two ways a stale password can end up being sent:

  • A saved password we pre-filled for you. To save you re-typing, TinyTax securely remembers the Gateway credentials from your last successful filing and pre-fills them next time. If you've since changed your Gateway password at HMRC, that saved one is now out of date. (We automatically discard the saved password after a failed attempt, so you'll be asked to enter it again — but the first try after a password change can still bounce with 1046.)
  • Your browser or password manager autofilling an old one. Independently of the above, your browser may silently drop a previously-saved password into the field, on top of — or instead of — what you typed.
In both cases the sign-in works fine on HMRC's own website, but the request reaching HMRC carries the stale password, so it returns Error 1046.

Signs this is the cause:

  • You filed successfully before, then changed your Gateway password, and now TinyTax keeps reporting Error 1046
  • The password works when you sign in directly on the HMRC website, but not here
  • The password field looked already filled when you opened the submission screen
To fix it: on the Review & Submit screen click Update Credentials & Retry, then click into the password box, clear it completely, and type your current Gateway password by hand — don't let your browser autofill it. Then submit.

ℹ Whatever is in the credentials box when you submit is exactly what TinyTax sends to HMRC. If a freshly-reset password works on HMRC's website but not here, make sure the password field holds your <em>new</em> password — clear it and type it by hand rather than relying on a saved or autofilled one.

If you don't know which of the five causes applies, work through them in this order — it's quickest:

  1. Wait at least 1 hour since your last attempt (in case it's a throttle).
  2. Go to the Business Tax Account sign-in page and choose "Sign in with Government Gateway" specifically — not the One Login option. Use the same 12-digit Gateway User ID and Gateway password you entered in TinyTax.
  3. If Gateway sign-in fails but One Login works → see Cause 3. You're entering One Login credentials when TinyTax needs Gateway ones.
  4. If Gateway sign-in fails outright → fix the credentials (Cause 1), then retry.
  5. If Gateway sign-in works but Corporation Tax isn't listed → enrol via the steps in Cause 2 (allow ~10 days for the activation code).
  6. If Gateway sign-in works and Corporation Tax is listed → open it and check the 10-digit UTR on that page against the UTR on your return before anything else. Do this even if you are sure the UTR is correct — see the warning under Cause 1.
  7. If the UTR matches → reset your Gateway password to clear any lockout (Cause 4), then retry in TinyTax. When re-entering, clear the password field and type the password by hand so browser autofill can't substitute a stale one (Cause 5).

What You Can Do From the Dashboard Right Now

Your figures are not lost. After a 1046, the affected period on the TinyTax dashboard shows a Review Error ▾ button. Open it and you have three options:

  1. View CT600 Error — re-opens the error panel with the credential-specific guidance above.
  2. Edit Form — drops you straight back into the multi-step form with every figure you've already entered pre-filled. Use this if you want to change a value (e.g. add an AIA capital allowance claim) before retrying.
  3. Review & Submit — takes you to the Review & Submit page so you can re-enter credentials and resubmit once you've fixed whichever of the four causes above applied.
Nothing was filed when 1046 fired — the return didn't reach HMRC — so editing and resubmitting is safe and won't create a duplicate.

Common Questions

I signed up for GOV.UK One Login — has my Gateway login been replaced?

No. GOV.UK One Login is a parallel sign-in system, not a replacement. Your old Government Gateway User ID and Gateway password still exist behind the scenes — they're just no longer the only way to sign into HMRC's website. The CT600 submission API hasn't been migrated to One Login yet, so TinyTax (and every other CT600 software) has to keep using the Gateway credentials. HMRC is expected to migrate the API in 2026–2027; until then, the Gateway login remains the path used for filing.

Where do I find my company's UTR if I don't have the paperwork?

Sign in to your Business Tax Account and click Corporation Tax. The 10-digit UTR appears on that overview page. You don't need any letters to hand.

It is also printed on any HMRC letter addressed to the company — the original "Corporation Tax — information for new companies" letter posted to the registered office shortly after incorporation, and on notices to deliver a return (form CT603).

If you can't see Corporation Tax in the Business Tax Account at all, the company isn't enrolled yet — see Cause 2.

Is my company UTR the same as my personal UTR?

No, and this catches people out regularly. If you file a Self Assessment return, you have a personal 10-digit UTR. The company has its own, entirely separate, 10-digit UTR for Corporation Tax. They look identical in format, so a personal UTR stored in a password manager or notes file years ago can be reused for company filings indefinitely without ever looking wrong. HMRC will reject it with Error 1046 and won't say why.

If you're diagnosing a 1046 and have never explicitly looked up the company's UTR, assume this is the cause until you've checked it against the Business Tax Account.

Why doesn't HMRC tell me which one is wrong?

HMRC returns Error 1046 deliberately as a generic response so it can't be used to probe valid User IDs. The downside is that the user has to rule out each cause manually.

I'm sure my credentials are right — can TinyTax just bypass this?

No. TinyTax sends your credentials to HMRC's Transaction Engine over their secure API. If HMRC says "no", we have no way to override that. The only fix is to satisfy HMRC's authentication.

How do I know if I'm enrolled for Corporation Tax?

Sign into your Business Tax Account. If "Corporation Tax" appears in the list of services, you are enrolled. If it doesn't, you aren't, and you must enrol before any online filing will succeed.

My company is brand new — what should I do?

Companies House notifies HMRC when a new company is incorporated, and HMRC posts a UTR to the registered office. Once you have the UTR, sign up for a Government Gateway account, then enrol for Corporation Tax (Cause 2). The full sequence — UTR letter → Gateway account → CT enrolment — typically takes 2–3 weeks for a new company.

Can I file my CT600 by post if I can't fix this?

Only in very limited circumstances. HMRC requires online filing for almost all companies. If you genuinely cannot enrol for Corporation Tax in time (e.g., the activation code is lost), call HMRC's Corporation Tax helpline on 0300 200 3410.

I keep retrying and it keeps failing — am I making it worse?

Yes — each rapid retry can extend the lockout. Stop retrying for at least an hour, work through the diagnosis above, and only attempt again once you've fixed the underlying cause.

Was this guide helpful?