Guides

Company Tax Return (CT600): Deadlines & Penalties

Published 9 Jun 2026 · Updated 18 Jul 2026

Your Company Tax Return (the CT600) goes to HMRC and is due 12 months after the end of your accounting period. But there's a catch most people miss: the tax itself has to be paid earlier — 9 months and 1 day after your period ends. This guide covers what the return includes, the deadlines, the late filing penalties (which went up in 2026), and the first-year quirk that can mean filing two returns.

A quick orientation: your Company Tax Return goes to HMRC, while your company accounts go to Companies House. Two filings, two places, two deadlines.

What is a Company Tax Return (CT600)?

A Company Tax Return isn't just the CT600 form. It normally consists of three things together:

• the completed CT600 form;
• your company accounts; and
• your Corporation Tax computations (showing how the figures on the return were worked out from the accounts).

All three are part of the return. Filing only the form doesn't count as a complete return — so it pays to have the full package ready.

When is the Company Tax Return due?

Note the order: you have to pay before you file. The 9-months-and-1-day rule applies to most smaller companies; large companies (profits over £1.5m) pay Corporation Tax by instalments on a different timetable instead. For the full detail — how the dates land from your year end, and the first-year split — see CT600 deadlines, or work out your exact dates with the corporation tax deadline calculator.

What happens if you file your tax return late?

HMRC charges a fixed penalty as soon as the return is late, and it grows the longer the return is outstanding. For returns with a filing date on or after 1 April 2026, that's £200 to start, rising to £400 past three months (higher again for repeat late filers), with tax-geared penalties added once a return is more than 6 and 12 months late. The fixed penalty applies even if your company made a loss or owes no tax.

None of it bites if the return is in on time. For the full penalty tables — the repeat-offender figures and the tax-geared percentages — see CT600 late filing penalties.

Do you need two tax returns in your first year?

Often, yes — and it surprises a lot of new directors. Your first accounts to Companies House usually cover more than 12 months (from incorporation to your accounting reference date). But a Corporation Tax accounting period can't be longer than 12 months, so HMRC splits that first long period into two accounting periods.

The result: one set of Companies House accounts, two CT600s, two payment dates. Each CT600 covers its own Corporation Tax accounting period, and the tax for each period has its own payment deadline, normally 9 months and 1 day after that period ends. Both returns share a single filing deadline where the period of account is no more than 18 months, but the first tax payment falls due earlier. It's worth getting this mapped out early in your first year.

How do you file a Company Tax Return?

You now file the CT600 using commercial software. HMRC's free online filing service closed on 31 March 2026, so since 1 April 2026 a self-filing company needs HMRC-recognised software (paper is only allowed with a reasonable excuse or for filing in Welsh). Your accounts and computations are filed in iXBRL format alongside the form.

Frequently asked questions

Do I need to file a tax return if my company made a loss? Yes. You still file the return — and recording the loss can let you reduce tax in other years.

Can I still file my accounts and tax return together for free? No. The free joint service closed on 31 March 2026. Accounts go to Companies House and the CT600 to HMRC separately — see what changed.

What's the difference between the filing deadline and the payment deadline? You pay your Corporation Tax 9 months and 1 day after your period ends, but you file the return up to 12 months after. Payment comes first.

Do I need an accountant to file a CT600? It isn't required, but Corporation Tax can get involved and mistakes are costly, so many companies get help. See do I need an accountant?

I'm a sole trader, CIC or charity — does this apply to me? The CT600 is the company tax return filed with HMRC. Sole traders file personal Self Assessment instead; a CIC does file a CT600 (it pays corporation tax); charities follow the charity tax regime. See other organisation types for your situation.

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