SimpleReturns

HMRC rejected your CT600: common reasons and fixes

Reviewed by Lee Jones, Founder · Updated 27 October 2026
The short answer

A rejection means HMRC's computer would not accept your submission. Nothing has been filed and nothing has been looked at. But your deadline does not move, and a late return costs £200 from the very first day. Rejections almost always come down to a handful of matching problems, and every one of them has a fix.

Official source. This guide is a plain-English summary of official GOV.UK guidance, not advice. The authoritative source is Company Tax Returns on gov.uk. Always rely on that over our summary.

What does a rejection actually mean?

Before HMRC's system accepts your Company Tax Return, it checks who is signing in, which company the return is for, and whether the whole package is there in the right format. If one of those checks fails, the whole submission is turned away. Think of it as a door that did not open, not a return that was marked wrong.

Two things follow. You are not in trouble. But until a submission is accepted, your company has not filed, so treat a rejection near the deadline as urgent.

What are the common causes, and how do I fix each one?

Work through these in order. The rejection message on screen usually points at one of them.

1. Your sign-in was refused (error 1046)

If the message mentions error 1046, "Authentication Failure" or details that "failed validation", the problem is the sign-in, not the return. It is nearly always a typo in your Government Gateway details, a wrong digit in your tax reference, or Corporation Tax sitting in the wrong account, such as an old accountant's.

The fix: stop retrying and follow our step-by-step guide to error 1046. It walks through each cause in order.

2. The wrong UTR on the return

Your company's UTR is its 10-digit tax reference. A wrong or missing digit usually gets the submission refused, but if the wrong number belongs to another company the return can go through against that company's record, and your company still has not filed.

The fix: check the number against a letter from HMRC, such as a notice to file or a payment reminder, not against memory. There should be exactly 10 digits. If you have no letter, your company can request a copy online and HMRC posts it to your registered address. Our guide on finding your company UTR shows you how.

3. The dates do not match what HMRC expects

HMRC already holds accounting period dates for your company. If the dates on your return do not match the period HMRC holds, the return is usually received but left unprocessed. HMRC may then change the dates it holds to match, or write to you to explain why the return cannot be accepted. This often happens after you change your company's year end, or in the first year, when HMRC's periods do not line up with your accounts. A period for Corporation Tax also cannot be longer than 12 months, so accounts covering more than a year need two returns, not one. So a dates mismatch may not show as a rejection on screen. If your dates might not match, look out for a letter from HMRC after you file.

The fix: sign in to your company's business tax account on gov.uk and check the exact accounting period dates HMRC holds. Make your return match those dates. If HMRC's dates are wrong, for example because you lengthened your financial year, contact HMRC to update them before your filing date rather than filing against the old dates.

4. HMRC is not expecting a return for that period

Your company files because HMRC sends it a notice to deliver a Company Tax Return. If you file for a period HMRC has not set up, the return is usually received but sits unprocessed until HMRC sets up the period or writes to you. So this one may not show as a rejection on screen either. If HMRC might not be expecting this return, look out for a letter from HMRC after you file.

The fix: check your business tax account for which periods HMRC is expecting, and file for the period on the notice. If you believe you owe a return but no period is open, ring HMRC and ask them to set it up.

5. Part of the return package was missing

A Company Tax Return is more than the CT600 form. Your company's accounts and the workings behind your tax figure travel with it, in a special computer-readable format. If a piece is missing or in the wrong format, the submission is refused. This is also why you cannot file on HMRC's website any more: the old free service has closed, and returns now almost always go through filing software that builds the package properly.

The fix: file through software that prepares and checks the full package for you rather than assembling files by hand. That is exactly the job SimpleReturns does.

What is at stake if I do not fix it in time?

Your deadline is 12 months after the end of the accounting period the return covers, and a rejection does not pause it. The penalties are blunt: £200 the first day the return is late, another £200 at 3 months, and at 6 and 12 months HMRC adds 10% of the unpaid tax each time. File late 3 times in a row and the £200 penalties become £1,000 each. If a rejection lands near your deadline, fix it the same day.

When should I ring HMRC?

Most rejections you can fix yourself with a letter from HMRC and ten careful minutes. Ring the Corporation Tax helpline on 0300 200 3410 if the dates HMRC holds for your company are wrong, if no period is open when you believe one should be, or if the rejection message simply does not match anything on this page. Have your company's UTR ready.

Common questions

Has anything been filed if my CT600 was rejected?

No. A rejection means HMRC's computer would not accept your submission. Nothing has been filed, nothing has been looked at, and your figures have not been judged. But until a submission is accepted your company has not filed, so treat a rejection near the deadline as urgent.

Does a rejection move my filing deadline?

No. Your deadline is 12 months after the end of the accounting period the return covers, and a rejection does not pause it. A late return costs £200 the first day, another £200 at 3 months, and at 6 and 12 months HMRC adds 10% of the unpaid tax each time.

Why is HMRC not expecting a return for my period?

Your company files because HMRC sends it a notice to deliver a Company Tax Return. If you file for a period HMRC has not set up, the return is usually received but sits unprocessed until HMRC sets up the period or writes to you, so it may not show as a rejection on screen. Check your business tax account for which periods HMRC is expecting, and file for the period on the notice. If the right period is not set up, ring HMRC to have it set up.

When should I ring HMRC about a rejection?

Ring the Corporation Tax helpline on 0300 200 3410 if the dates HMRC holds for your company are wrong, if no period is open when you believe one should be, or if the rejection message does not match any of the common causes. Have your company's UTR ready.

Would rather not meet the rejection screen at all?

SimpleReturns checks your return against HMRC's rules before anything is sent: the reference, the dates, the complete package. If something would be refused or held up, we tell you in plain English what needs fixing first. It is free to start, no card needed, and one payment of £99 with Guided covers both filings, your tax return to HMRC and your accounts to Companies House.

Start your return

And if the fix is one only HMRC can make, like lengthening the accounting period they hold, we say so plainly, because you have to contact HMRC for that yourself.

General guidance, not advice. This guide explains how the rules generally work for small UK limited companies. It isn't tax advice for your specific situation, if you're unsure, check with us or an accountant.