When Is My Company Tax Return (CT600) Due?

Every UK limited company must file a company tax return with HMRC — even if it made a loss or owes no tax. This return, officially called the CT600, has a strict deadline, and missing it triggers automatic penalties from the day after it passes.

This guide explains when your CT600 is due, how to calculate your specific deadline, and what happens if you file or pay late.

The CT600 Filing Deadline

Your company tax return must be filed with HMRC within 12 months of the end of your accounting period.

For example:

• Accounting period ends 31 March 2025 → CT600 due by 31 March 2026
• Accounting period ends 31 December 2024 → CT600 due by 31 December 2025
• Accounting period ends 30 September 2024 → CT600 due by 30 September 2025

HMRC does not offer a grace period. Automatic penalties begin the day after the deadline, regardless of whether you owe any tax.

The Corporation Tax Payment Deadline

Your corporation tax payment deadline is different from your filing deadline — and it falls earlier.

Corporation tax must be paid by 9 months and 1 day after the end of your accounting period.

For example:

• Accounting period ends 31 March 2025 → Tax due by 1 January 2026
• Accounting period ends 31 December 2024 → Tax due by 1 October 2025

This means most companies pay their tax bill before they formally file the return. You will need to calculate your liability and pay it, then later file the CT600 confirming those figures. If you overpay, HMRC will refund the difference once the return is processed.

What Is an Accounting Period?

Your accounting period is the span of time covered by your company tax return. It is usually the same as your company's financial year — typically 12 months.

However, accounting periods can be shorter than 12 months in some situations:

• New companies: The first accounting period runs from the date of incorporation to the end of the first financial year, which is often less than 12 months
• Year-end changes: If you change your company's year-end date, the transitional period may be shortened
• Long periods: HMRC will not accept an accounting period longer than 12 months. If your financial year spans more than 12 months, it must be split into two separate accounting periods, each requiring its own CT600

For guidance on first-year filings, see our guide to filing your first company tax return.

Your First CT600: When Is It Due?

HMRC registers new companies for corporation tax automatically when they are incorporated. You will receive a letter with your Unique Taxpayer Reference (UTR) and the start date of your first accounting period.

Your first CT600 is due 12 months after your first accounting period ends — not 12 months after you incorporated. The exact date depends on when your year-end falls, so log into your HMRC business tax account or check your Notice to Deliver a Return to confirm your specific deadline.

For a step-by-step walkthrough of the process, read our guide on how to file your company tax return.

What Happens If You File Late?

Late CT600 filing attracts automatic penalties, regardless of whether any tax is owed.

Days late	Penalty
1 day	£100 automatic fine
3 months	Additional £100 fine
6 months	10% of unpaid tax (if applicable)
12 months	A further 10% surcharge or higher

A dormant or loss-making company is just as liable for the £100 penalty as a profitable one — the penalty is triggered by late filing, not by unpaid tax. Habitual late filing also attracts higher flat-rate penalties: companies that file late in three consecutive periods face increased charges at the 1-day and 3-month marks.

For the full penalty schedule and how to appeal, see our guide to CT600 late filing penalties.

What Happens If You Pay Corporation Tax Late?

Late payment triggers interest charges on the outstanding amount. HMRC calculates interest daily from the day after the payment deadline until you pay. Even if you file your return on time, interest accrues on any tax not paid by the 9 months and 1 day deadline.

For more on how these charges work, see our guide to corporation tax interest and late payment charges.

If you cannot pay on time, contact HMRC before the deadline to discuss a Time to Pay arrangement. This allows you to spread payments over an agreed period, which can reduce the total interest charged compared with simply paying late.

Extended Accounting Periods: Two Returns Required

If your accounting year runs for more than 12 months — for example, if your first year spans 15 or 16 months — HMRC will not accept a single CT600 covering the full period.

You must file two separate CT600 returns:

1. The first return covers months 1 to 12
2. The second return covers the remaining months

Each return has its own filing and payment deadlines, calculated from the end of its respective accounting period. Your profits are apportioned between the two periods on a time basis. If you are unsure how to split the periods, accounting software designed for corporation tax will typically handle this automatically.

When Are My Company Accounts Due?

Your annual accounts for Companies House have a separate deadline from the CT600. For a private limited company, accounts must normally be filed within 9 months of your accounting period end.

This means the accounts deadline and the corporation tax payment deadline often fall in the same month — with the CT600 filing deadline coming three months later. Understanding how these deadlines interact helps you plan your year-end workload efficiently. See our guide to company accounts filing deadlines for more detail.

How to Work Out Your Deadline

To calculate your CT600 filing deadline:

1. Find your accounting period end date (your company's year-end)
2. Add exactly 12 months — that is your filing deadline

To find your corporation tax payment deadline:

1. Take the same year-end date
2. Add 9 months and 1 day — that is when payment is due

You can verify these dates by logging into your HMRC business tax account, or by checking the Notice to Deliver a Return that HMRC issues after each year-end.

Keeping Track of Your Deadlines

Missing a CT600 deadline is easily avoided with good diary management. A few practical steps:

• Log your accounting period end date and both deadlines (payment and filing) in your calendar as soon as each year-end passes
• Set reminders 30 days and 7 days before each deadline
• Use our company tax return checklist to make sure all figures are ready well before the filing date
• Consider filing early — once you have final figures, there is no benefit in waiting

Filing early also means HMRC processes any repayment owed to you sooner, which is particularly useful if you have overpaid based on an estimated liability.

Summary

Your CT600 must be filed with HMRC within 12 months of your accounting period end. Corporation tax must be paid earlier — by 9 months and 1 day after the same date. Missing either deadline results in automatic penalties or interest charges. Calculate your deadlines by adding the relevant period to your year-end date, verify them through your HMRC business tax account, and file early where possible to avoid last-minute complications.