The CT600 is the UK Company Tax Return that a limited company files with HMRC to report its taxable profit and work out its Corporation Tax. It's due 12 months after the end of the accounting period it covers, but the tax itself is usually due earlier, 9 months and one day after the period ends. You must file it even if the company made a loss or owes nothing.
Two deadlines, a few months apart, are what catch new directors out. Everything below comes from GOV.UK's Company Tax Returns guide and HMRC's CT600 publication page, checked in October 2026.
Who has to file a CT600
- Any company or association that receives a notice to deliver a Company Tax Return from HMRC.
- Companies with a loss or no Corporation Tax to pay still file.
- Sole traders and partnerships don't: they file Self Assessment returns instead.
The profit on the CT600 isn't the same as the profit in your annual accounts. The return works out profit for Corporation Tax, which adjusts the accounts for things the tax rules treat differently.
The two deadlines
| What | Deadline |
|---|---|
| Pay Corporation Tax (most companies) | 9 months and one day after the end of the accounting period |
| File the CT600 | 12 months after the end of the accounting period |
| Amend a filed return | Usually within 12 months of the filing deadline |
So for a period ending 31 March, the tax is usually due by 1 January and the return by 31 March the following year. Preparing the return early is the simplest way to know the bill before it's due. The rates and Marginal Relief rules that set the bill are covered in Corporation Tax UK.
How to file it
HMRC says you should use commercial software to file annual accounts and Company Tax Returns. The paper CT600 can only be used to amend a return you've already submitted, or to file on paper if you have a reasonable excuse or are filing in Welsh. A paper return goes to Corporation Tax Services, HMRC, BX9 1AX, with form WT1 if HMRC's online services stopped you filing because of an IT problem.
You can ask an accountant to prepare and file it or do it yourself. To let HMRC talk to your accountant, you use form 64-8.
Supplementary pages
| Page | Used for |
|---|---|
| CT600A | Close company loans to participators and arrangements conferring benefits |
| CT600B | Controlled foreign companies and foreign permanent establishment exemptions |
| CT600C | Group and consortium relief |
| CT600E | Charities and Community Amateur Sports Clubs |
| CT600L | Research and development |
| CT600M | Freeports and Investment Zones |
| CT600N | Residential Property Developer Tax |
| CT600P | Creative industries |
CT600A is the one most small owner-managed companies meet: it's where a loan from the company to a director shows up, which is why an overdrawn director's loan account needs sorting before the year end.
A detail from the form itself: it already carries Northern Ireland boxes in readiness for a devolved rate, and HMRC says not to make any entries in them until a Northern Ireland rate has been introduced.
Late filing penalties
| Time after the filing deadline | Penalty |
|---|---|
| 1 day | £200 |
| 3 months | Another £200 |
| 6 months | HMRC estimates your Corporation Tax and adds 10% of the unpaid tax |
| 12 months | Another 10% of any unpaid tax |
If your return is late 3 times in a row, the £200 penalties rise to £1000 each, as of October 2026. At 6 months, HMRC sends a tax determination of what it thinks you owe, and you can't appeal against it: you have to pay and file. You can appeal a penalty online with a reasonable excuse, but only after you've filed the return.
Getting it right
- Diary both dates as soon as your accounting period ends: payment at 9 months and one day, filing at 12 months.
- Prepare the accounts first, then the tax computation that adjusts them for Corporation Tax.
- Add any supplementary pages, such as CT600A for director loans.
- File with commercial software, using the company's UTR.
- If you find a mistake, amend within 12 months of the filing deadline.
If you think the company doesn't need to file
The duty to file starts with the notice to deliver a Company Tax Return. A sole trader or partnership never files a CT600, but a limited company that has received a notice files even with no profit. If you can't file online because of an IT problem with HMRC's online services, you send the paper return with form WT1. Supplementary pages beyond the main form are only needed when they apply. A company that has stopped trading or never started still has duties, which the guide to a dormant company in the UK covers.
Appealing a penalty has its own checklist: you'll need the company's UTR, the date on the penalty notice, the penalty amount, the end date of the accounting period, and an explanation of why the return was late. The appeal form has to be completed in one go, and you print it and post it to the address on the form.
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The verdict
Treat the payment date, not the filing date, as your real deadline, because the tax is usually due 3 months before the return. File every year even with a loss, and don't let it slip three times: the penalties jump from £200 to £1000 each.
Company tax returns take time, so this guide to AI tax software is worth a look if you want help with the workings.
Frequently asked questions
What is a CT600?
It's the UK Company Tax Return a company files with HMRC to report its profit for Corporation Tax and work out the tax due.
When is the CT600 due?
12 months after the end of the accounting period it covers. Corporation Tax is usually payable 9 months and one day after the period ends.
Must a company file a CT600 if it made a loss?
Yes. You must still send a return if you make a loss or have no Corporation Tax to pay.
Can I file a CT600 on paper?
Only to amend a return already submitted, or if you have a reasonable excuse or are filing in Welsh. HMRC says to use commercial software.
What is the penalty for a late CT600?
£200 at 1 day, another £200 at 3 months, then 10% of unpaid tax at 6 months and another 10% at 12 months.
Where do I send a paper CT600?
To Corporation Tax Services, HMRC, BX9 1AX, along with form WT1 if an IT problem with HMRC online services stopped you filing online.
Can I appeal a CT600 late filing penalty?
Yes, if you have a reasonable excuse, but you must file the Company Tax Return before you appeal.
