If you run a UK limited company, you've probably seen "CT600" mentioned and not been entirely sure what it means, beyond "something to do with Corporation Tax." Here's what it actually is, who has to file one, and the two deadlines people most often mix up.
The short version: CT600 is the Company Tax Return you file with HMRC to report your profits and Corporation Tax owed. Corporation Tax is normally paid 9 months and 1 day after your accounting period ends; the CT600 return itself is normally due later, 12 months after the accounting period ends. It must be filed using commercial software, not by hand.
What CT600 actually covers
A CT600 is a Company Tax Return: it tells HMRC how much profit your company made and how much Corporation Tax it owes on that profit. A complete submission isn't just the CT600 form itself. It also includes your statutory accounts and detailed tax computations, formatted in iXBRL so HMRC can process it automatically, plus any supplementary schedules relevant to your company (for example, if you have capital allowances or losses to carry forward).
You need to file one even in some cases where your company made no profit, and, depending on your circumstances, even while dormant. If you're not sure whether your company needs to file, that's worth checking directly rather than assuming a dormant company is automatically exempt.
The two deadlines people mix up
This is the single most common source of confusion, because there are genuinely two separate deadlines running on different clocks from the same accounting period end date:
- Paying the tax: normally due 9 months and 1 day after your accounting period ends.
- Filing the CT600 return itself: normally due 12 months after your accounting period ends, three months later than the payment deadline.
In other words, you have to pay what you owe before you're required to have actually filed the paperwork confirming the figure. That surprises a lot of first-time company owners. Missing either deadline can trigger penalties and interest separately, so it's worth tracking them as two dates, not one.
Why you can't just fill it in yourself on a form
HMRC's older free joint filing service, which let smaller companies file a CT600 directly through a basic online form alongside accounts, has closed. Company Tax Returns now have to go through HMRC-recognised commercial software, the same iXBRL-based route that Companies House is also moving all accounts filing to by 2028. That's not a change specific to CT600: it reflects a broader shift toward every UK filing route running through structured, machine-readable software submissions rather than manual forms.
Who needs to think about this
- Dormant companies, who may still need to confirm dormant status correctly with HMRC.
- Micro-entity and small trading companies, who need a CT600 every accounting period once trading starts.
- Anyone whose accountant has told them "we need software for this now": this is usually what they mean.
Digital Filing Systems is building Corporation Tax filing software: CT600, iXBRL accounts and computations, HMRC validation and acknowledgement handling, as our first product. It's currently in development; see its current status.
Sources
- HMRC: Corporation Tax commercial software suppliers — gov.uk
- Company Tax Returns: overview — gov.uk
- Pay Corporation Tax — gov.uk