This one is short, practical, and it will affect thousands of small companies who have no idea it has happened — because it only becomes apparent when they sit down to file, and find the door locked.
What has happened?
HMRC’s free Company Accounts and Tax Online (CATO) service closed on 31 March 2026.
For years, it allowed small, straightforward companies to file their accounts to Companies House and their CT600 to HMRC in one place, at no cost. A great many owner-managed companies used it, either themselves or through a bookkeeper.
It is gone. Every UK company must now use commercial software to file its corporation tax return.
What are my options?
Buy your own software licence. Perfectly viable, but note that filing a CT600 is not merely data entry — the computation is where the tax is either saved or wasted. Software will happily file a return that is technically valid and financially poor.
Use an accountant. Most firms include the software as part of a compliance fee, so there is nothing separate to buy, learn or configure. You also get the computation reviewed by someone qualified, which is the part that actually has value.
What you cannot do is nothing. The filing obligation has not changed — only the route to satisfying it.
While you’re at it — check your dates
Since you are being forced to think about this anyway, it is worth checking the two deadlines people most often confuse:
Corporation tax is payable nine months and one day after the end of your accounting period.
The CT600 return is due twelve months after the end of the period.
The money is due three months before the paperwork is — which remains the single most common cash-flow shock for owner-managed companies, and it is entirely avoidable with a forecast.
Separately, your accounts are generally due at Companies House nine months after your accounting reference date, with automatic penalties from £150 to £1,500 for filing late, doubling if you are late two years running.
Frequently asked questions
Can I still file my corporation tax return for free?
No. HMRC’s free Company Accounts and Tax Online service closed on 31 March 2026. All companies must now file the CT600 using commercial software — either your own licence, or through an accountant who holds one.
What happens if I miss the CT600 deadline?
HMRC applies penalties starting at £100 for a return one day late, increasing with further delay and doubling for repeated lateness. Interest also runs on any unpaid tax from the due date — which was three months earlier, since payment falls due before the return.
When is my corporation tax actually due?
Nine months and one day after the end of your accounting period. The return itself is not due until twelve months after the period end — so the money is due three months before the paperwork. Larger companies pay by quarterly instalments instead.