What does a rejection actually mean?
Before HMRC's system stores your Company Tax Return, it checks that everything lines up: who is signing in, which company the return is for, which dates it covers, and whether the whole package is there. If any one of those checks fails, the entire 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. One swapped or missing digit and HMRC cannot match the return to your company, so the submission is refused.
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 is expecting, the return can be turned away. 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.
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 try to file for a period HMRC has no notice open for, the system has nothing to match your return against.
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 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.