- Corporation Tax filing is done by submitting a Company Tax Return, also known as a CT600, to HMRC.
- A complete Company Tax Return includes the CT600, statutory accounts, and tax computations in iXBRL format.
- The filing deadline is 12 months after the end of your accounting period, even if your company made a loss.
Corporation tax filing means submitting your company tax return (the CT600) to HMRC and filing your annual accounts with Companies House. These are two separate obligations, and most limited companies need to complete both every year.
You can file the CT600 yourself through HMRC-recognised software, or hand the whole job to an accountant like Sleek’s accounting and tax filing service.
The return is due 12 months after your accounting period ends. The tax, though, is due earlier, and missing either deadline now costs more than it used to.
How do you file your corporation tax return (CT600)?
You file your corporation tax return by completing form CT600 and submitting it online to HMRC, alongside your company accounts and tax computations. Paper returns are not accepted for active trading companies.
The process runs in five steps:
- Prepare your statutory accounts for the accounting period.
- Work out your taxable profit and your corporation tax computation.
- Complete the CT600 using HMRC-recognised commercial software.
- Submit the CT600 to HMRC, with accounts and computations attached in iXBRL format.
- File your accounts separately with Companies House.
One change worth flagging: HMRC closed its free CATO (Company Accounts and Tax Online) filing tool on 31 March 2026. All CT600 returns must now be filed in iXBRL format through commercial software, so if you used the old free service, you’ll need a new tool in place before your next deadline.
What must you file, and to whom?
Corporation tax filing involves two filings to two different bodies, and it helps to keep them clearly separate in your head.
Filing | Goes to | What it covers |
CT600 (company tax return) | HMRC | Your taxable profit, reliefs, and corporation tax due |
Annual accounts | Companies House | Your company’s financial statements for the year |
Tax computations | HMRC (with the CT600) | The workings behind your profit figure |
Your accounts are usually filed in both places: a full version to HMRC with the CT600, and a version (sometimes abbreviated for small companies) to Companies House. Even a dormant or loss-making company that HMRC expects a return from must still file a CT600.
When is your corporation tax filing deadline?
Your CT600 filing deadline is 12 months after the end of your accounting period. So if your accounting period ends on 31 March 2026, your return is due by 31 March 2027.
Here’s the trap. The deadline to pay corporation tax falls earlier than the deadline to file the return. For most companies, payment is due nine months and one day after the accounting period ends. Filing comes three months after that.
Obligation | Deadline (for a 31 March 2026 year end) |
Pay corporation tax | 1 January 2027 |
File CT600 | 31 March 2027 |
Because the payment date comes first, you often need to know your tax bill before the return is formally due. For more on the payment side, see our guide to paying corporation tax.
What rates feed into your corporation tax return?
The rates that determine your bill for 2026/27 depend entirely on your taxable profit. There are three bands, and they’re unchanged from the structure introduced in April 2023.
Taxable profit | Rate |
£50,000 or less | 19% (small profits rate) |
£50,001 to £250,000 | Marginal relief (effective rate tapers between 19% and 25%) |
Over £250,000 | 25% (main rate) |
Marginal relief uses a fraction of 3/200, which your filing software calculates automatically. In practice, each extra pound of profit in the £50,000 to £250,000 band is taxed at an effective marginal rate of 26.5%, slightly higher than the headline 25%.
The thresholds are reduced if you have associated companies or a short accounting period. For a fuller breakdown, see our explainer on corporation tax rates.
if your profits sit near £50,000, the timing of income and allowable expenses across your year end can keep you in the lower band. It's worth modelling before your accounts are finalised.
How do you actually file it?
You have three realistic routes, and the right one depends on how complex your accounts are and how confident you feel with the software.
- File it yourself using HMRC-recognised commercial software. This suits straightforward companies, but you’ll handle the iXBRL tagging and computations.
- Use accounting software that prepares the CT600 from your bookkeeping data, reducing manual rekeying and the risk of tagging errors.
- Hand it to an accountant, who prepares the accounts, the computation, and the return, then submits everything for you.
Whichever route you choose, remember that a return rejected by HMRC’s validation rules due to incorrect iXBRL tagging does not count as filed. If the error isn’t fixed before the deadline, the late penalty applies.
Setting up new software well ahead of your deadline, rather than the week it’s due, is the safest approach. This is a common decision point where directors weigh the cost of reducing their corporation tax against the time filing takes, and bringing in support often pays for itself.
What are the late-filing penalties for a CT600?
The penalties for filing your CT600 late doubled on 1 April 2026, the first increase since 1998, and they now escalate quickly. These apply even if your company owes no corporation tax at all.
How late | Penalty (from 1 April 2026) |
1 day late | £200 fixed |
3 months late | A further £200 (£400 total) |
6 months late | HMRC estimates your bill and adds 10% of the unpaid tax |
12 months late | A further 10% of the unpaid tax |
If your company files late for three accounting periods in a row, the fixed penalties rise to £1,000 per return, or £2,000 if each return is more than three months late. Companies House also runs its own separate penalty regime for late accounts, so a single missed year end can trigger fines from both bodies at once.
You can read more on how these stack up in our guide to HMRC and Companies House fines.
Is MTD for Corporation Tax here yet?
There is no Making Tax Digital for Corporation Tax, and there is no longer a planned rollout date. In its July 2025 Transformation Roadmap, HMRC confirmed it does not intend to introduce MTD for Corporation Tax.
This is a genuine change from earlier expectations, when limited companies were told to prepare for quarterly digital reporting. That requirement has been formally dropped. You’ll continue filing your CT600 annually through the existing Corporation Tax Online process, with no quarterly updates and no MTD-specific software mandate for corporation tax.
Two things to keep in mind, though. If your company is VAT-registered, MTD for VAT still applies. And if you have personal sole trader or rental income over £50,000 alongside your company, MTD for Income Tax may affect you personally from April 2026, even though it doesn’t touch your corporation tax.
How Sleek helps with corporation tax filing
Filing corporation tax correctly means juggling two deadlines, two bodies, and a computation that has to be right before the payment date even arrives. For a compliance-anxious director, that’s a lot to track alongside running the business.
Sleek prepares your accounts, builds your corporation tax computation, and files both your CT600 with HMRC and your accounts with Companies House. Deadlines are tracked centrally, so you’re never exposed to the new doubled penalties through a missed diary date.
Disclaimer: The preceding information is not legal advice. This content is aimed to provide general guidance. For more formal or legal advice, contact Sleek directly.
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FAQs on corporation tax filing
Do I need to file a CT600 if my company made a loss?
Yes. If HMRC expects a return from your company, you must file a CT600 even where there’s no corporation tax to pay. Loss-making and dormant companies are both included. Filing the return on time also lets you record losses you may want to carry forward or back against profits in other periods, which protects future relief.
Can I file my corporation tax return myself without an accountant?
Yes, you can file a CT600 yourself using HMRC-recognised commercial software. It’s manageable for simple companies, but you’ll be responsible for the tax computation and the iXBRL tagging. Companies with associated entities, complex reliefs, or property income often find the risk of error outweighs the saving, and choose to use an accountant instead.
What happens if my CT600 is rejected by HMRC?
A return rejected by HMRC’s validation rules has not been filed, even if you submitted it before the deadline. This usually happens through incorrect iXBRL tagging or a mismatched taxonomy. If you don’t correct and resubmit it before your deadline, the £200 day-one penalty applies, so always file with enough time to fix a rejection.
Is the CT600 filing deadline the same as the payment deadline?
No. The CT600 must be filed within 12 months of your accounting period ending, but the tax is due nine months and one day after the period ends. The payment deadline comes first, which surprises many directors. You effectively need to know your tax figure before the return itself is formally due.
Do I file my accounts with HMRC or Companies House?
Both. You file a full set of accounts to HMRC alongside your CT600, and a set to Companies House separately. Small companies can often file an abbreviated or filleted version with Companies House. The two filings have their own deadlines and submission systems, so neither one covers the other.
What is iXBRL and why does it matter for filing?
iXBRL is a digital tagging format that HMRC requires for company accounts and computations submitted with the CT600. Since HMRC closed its free CATO filing tool on 31 March 2026, all returns must be filed in iXBRL through commercial software. Incorrect tagging can cause HMRC to reject your return, so the format matters for getting your filing accepted.
Will MTD for Corporation Tax ever be introduced?
There’s no confirmed plan. HMRC stated in its July 2025 Transformation Roadmap that it does not intend to introduce MTD for Corporation Tax. It may continue improving the existing online service over time, such as pre-populating some data, but there’s no mandatory quarterly reporting framework on the horizon for corporation tax.
