Key takeaways
- A company must file a CT600 Company Tax Return within 12 months of the end of each accounting period, even if it made a loss or owes no tax.
- The return includes the CT600 form, the company's accounts and a tax computation, all in iXBRL format.
- HMRC's free online filing service has closed, so returns are now filed through commercial software or an accountant.
- Late filing penalties doubled for returns due from 1 April 2026: £200 at once and another £200 after three months.
- Payment is due earlier than filing: usually 9 months and 1 day after the end of the accounting period.
Every UK limited company that receives a notice to deliver a Company Tax Return must file one for each accounting period. The return tells HMRC the company’s taxable profits and the Corporation Tax due. This guide covers what is in it, how to file in 2026 and the deadlines that matter.
Who must file
A company must file if HMRC sends a notice to deliver a Company Tax Return, which it does for almost all active companies. You must file:
- even if the company made a loss
- even if no Corporation Tax is due
- for each accounting period, which can be no longer than 12 months
Dormant companies may be told they do not need to file; if you have a notice to file, you must still do so unless HMRC agrees otherwise. See what is a dormant company.
What the return contains
| Part | What it is |
|---|---|
| CT600 form | The return itself: company details, profits, Corporation Tax calculation, reliefs claimed |
| Statutory accounts | The company’s accounts for the period (the same accounts filed at Companies House, though small companies may file filleted accounts there) |
| Tax computation | Shows how accounting profit is adjusted to taxable profit: disallowed expenses added back, capital allowances deducted, losses used |
| Supplementary pages | Where needed, for example CT600A for loans to participators, CT600L for R&D credits, CT600C for group relief |
The accounts and computation must be in iXBRL (tagged) format. Commercial software produces this automatically.
How to file
HMRC’s free online service for filing small companies’ returns and accounts together has closed. Companies now file through:
- commercial software that files the CT600, accounts and computations to HMRC, often with accounts filing to Companies House as well
- an accountant using their own software
Paper returns are only accepted in limited cases, such as a reasonable excuse for not filing online or filing in Welsh.
Deadlines
| What | Deadline |
|---|---|
| Pay Corporation Tax (profits up to £1.5 million) | 9 months and 1 day after the end of the accounting period |
| Pay Corporation Tax (large companies) | Quarterly instalments; see instalment payments |
| File the CT600 | 12 months after the end of the accounting period |
| File accounts at Companies House | 9 months after the end of the financial year for private companies (first accounts: 21 months from incorporation) |
The payment deadline comes before the filing deadline. Most companies prepare the return in time to pay the right amount. See Corporation Tax payment deadlines.
Late filing penalties
For returns due on or after 1 April 2026:
| How late | Penalty |
|---|---|
| 1 day | £200 |
| 3 months | Another £200 |
| 6 months | HMRC estimates the tax and charges 10% of the unpaid tax |
| 12 months | Another 10% of any unpaid tax |
If a return is late three times in a row, the fixed penalties rise to £1,000 each (£2,000 at three months). For returns due before 1 April 2026 the fixed penalties were half these amounts: £100 each time, or £500 for a third consecutive late return. If a return is six months late, HMRC can issue a determination of the tax due, which you cannot appeal; the only way to replace it is to file the return.
Amending a return
You can amend a CT600 within 12 months of the filing deadline, using your software. After that, overpaid tax can be reclaimed through an overpayment relief claim, and underpaid tax should be disclosed to HMRC.
Getting the computation right
Common adjustments in a tax computation:
- add back depreciation, client entertaining, fines and penalties, and private expenditure
- deduct capital allowances, including the £1 million Annual Investment Allowance and full expensing; see capital allowances
- apply losses brought forward or claim group relief; see loss relief
- claim reliefs, such as R&D; see R&D tax relief
- check the rate, including marginal relief and the effect of associated companies
Frequently asked questions
When is the CT600 due?
12 months after the end of the accounting period. The tax itself is usually due 9 months and 1 day after the period ends.
Can I still file my company tax return free with HMRC?
No. HMRC’s free joint filing service has closed, so you need commercial software or an accountant to file.
What is the penalty for filing a CT600 late?
For returns due from 1 April 2026, £200 immediately and another £200 after three months, plus tax-geared penalties after six and twelve months.
Do I need to file a Company Tax Return if the company made a loss?
Yes. If HMRC has sent a notice to file, you must file even if there is a loss or no tax to pay.
Sources
Rates, thresholds and rules in this guide were checked against HMRC and GOV.UK guidance in October 2026. Tax rules change, so check the current position on GOV.UK or with a qualified adviser before acting. This guide is general information, not personal advice.