CT600 Deadline: When Is Your Company Tax Return Due?

Your CT600 Company Tax Return must reach HMRC by 12 months after the end of your accounting period. Miss that date and automatic penalties start from day one — regardless of whether you owe any tax.

There is a second, earlier deadline to know: Corporation Tax must be paid by 9 months and 1 day after the accounting period ends — three months before the filing deadline. Most directors are surprised to learn they need to pay before they file.

Filing Deadline vs Payment Deadline

These are two separate obligations with two separate deadlines:

Obligation	Deadline
Pay Corporation Tax	9 months and 1 day after period end
File CT600 return	12 months after period end

Example: Your company's accounting period ends on 31 March 2025.

Pay by: 1 January 2026
File by: 31 March 2026

If you pay late, HMRC charges interest on the overdue amount. Since 9 January 2026, the late payment rate is 7.75% per year. Unlike late filing penalties, this interest is a deductible expense for Corporation Tax purposes.

What Is a CT Accounting Period?

A Corporation Tax accounting period runs for a maximum of 12 months. It usually aligns with your company's financial year — the period your statutory accounts cover.

Your accounting period normally:

Starts when your company first becomes liable to Corporation Tax (usually on incorporation, or when it starts trading)
Ends on the last day of your financial year, or earlier if you change your year-end
Cannot exceed 12 months for CT purposes

HMRC will tell you your accounting period dates when you register for Corporation Tax. You can also confirm them in your HMRC online business tax account, or on the form CT603 (notice to deliver a return) that HMRC sends you.

Accounting Periods Longer Than 12 Months

If your company's financial year is longer than 12 months — most commonly in the first year after incorporation — you cannot file a single CT600 covering the whole period. You must file two separate CT600 returns:

First return: covers the first 12 months
Second return: covers the remaining period (up to 12 months)

Each return has its own 12-month filing deadline, counting from the end of that return's period. You will need to apportion income, expenses, and capital allowances between the two periods manually — HMRC does not do this for you. The apportionment is usually done on a time basis unless the accounts show a clearer split.

Dormant Companies

A dormant company is not automatically exempt from filing. The rules are:

If HMRC issues you a notice to deliver a CT600 (form CT603), you must file one — even for a dormant period.
If you notify HMRC that your company is dormant and HMRC accepts this, they should stop issuing notices. Until you have HMRC's confirmation, comply with any notices you receive.
A dormant nil return is still a valid CT600 — you file it showing no income, no tax, and zero liability.

Note that HMRC's definition of dormant for Corporation Tax can differ from Companies House's definition. A company can be dormant for Companies House purposes but still have a Corporation Tax obligation (for example, if it earns bank interest).

Penalties for Late Filing

Filing your CT600 after the deadline triggers automatic penalties — no warning, no grace period. From 1 April 2026, HMRC increased the fixed-rate penalties. The regime that applies depends on your return's filing due date, not when you actually file:

Returns with a filing due date before 1 April 2026

Delay	Penalty
1 day late	£100
More than 3 months late	Additional £100
More than 6 months late	10% of unpaid tax (based on HMRC's estimate)
More than 12 months late	Additional 10% of unpaid tax
Third consecutive late return	£500 initial + £500 at 3 months

Returns with a filing due date on or after 1 April 2026

Delay	Penalty
1 day late	£200
More than 3 months late	Additional £200
More than 6 months late	10% of unpaid tax (based on HMRC's estimate)
More than 12 months late	Additional 10% of unpaid tax
Third consecutive late return	£1,000 initial + £1,000 at 3 months

The tax-geared penalties (10% at 6 months, another 10% at 12 months) apply under both regimes. If you have not filed when these trigger, HMRC uses an estimate of your tax liability to calculate the amount. These can be significant for profitable companies.

The fixed-rate penalties apply even if you owe no tax at all. There are no daily accruing penalties for Corporation Tax (those apply to Self Assessment, not CT).

How to Find Your Exact Deadline

Check your CT603 — the notice to deliver a return that HMRC sends you states the accounting period and filing deadline.
Log in to HMRC's business tax account at www.gov.uk/business-tax-account — your Corporation Tax deadlines appear on the dashboard.
Calculate it yourself — count 12 months from the last day of your accounting period. If the period ends on 31 March 2025, the filing deadline is 31 March 2026.

If you are unsure of your accounting period end date, check your statutory accounts or your HMRC online account.

Requesting More Time to File

HMRC can grant an extension to the filing deadline in exceptional circumstances. In practice, extensions are rarely granted and must be requested before the deadline passes. Inability to pay is not grounds for an extension — you should file on time even if you cannot pay, because late filing penalties stack on top of any payment interest.

Filing Your CT600

HMRC's free Corporation Tax filing service closed on 31 March 2026. Limited companies must now use commercial filing software or an agent to submit CT600 returns electronically.

Taxpipe is an online service for small UK limited companies. Enter your income, expenses, and capital allowances through a guided wizard — we generate the CT600, iXBRL accounts, and iXBRL tax computations, then submit directly to HMRC using your own Government Gateway credentials. A Government Gateway account enrolled for Corporation Tax is required. The process takes around 15 minutes for a straightforward return.

Don't miss your deadline

Check your deadline, then file with Taxpipe. Our guided wizard takes about 15 minutes, and we submit directly to HMRC. £59, one-time fee.

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CT600 Deadline: When Is Your Company Tax Return Due?