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When is my CT600 due, and what goes with it?

Usually 12 months after your period ends.

For a year ending 31 December 2026, your Corporation Tax return is due by 31 December 2027. It can be later if HMRC serves its notice to file late. Your tax is due about 3 months before the return.

Published

At a glance

Applies to
Your company, once HMRC sends it a "", even if it owes no tax
Deadline
Usually 12 months after the end of your
What you send
, your accounts and your , online
If you are late
For returns due on or after 1 April 2026, a £200 penalty from the first day, rising the later you are

Sources

This entry is general information for directors of UK private limited companies, not advice about a particular company.

Does this apply to me?

  • HMRC has sent your company a notice to file a return. Yes: you must file, even if your company made a loss or owes no tax.

  • You have told HMRC your company is dormant. No, unless HMRC asks for a return or your company starts again.

  • Your company is dormant, but you have not told HMRC. Yes, once HMRC sends a . HMRC says a return is still required, and it may charge penalties if you do not file.

  • HMRC sent a notice for a period when your company was dormant. Yes: file online for that period. The return shows HMRC your company was .

Sources

What to do

  1. Find the end of your accounting period. This is the period your return covers. It is 12 months at most, and usually the same as your .

  2. Find your deadline. It is usually the same date 12 months later. So 31 December 2026 gives 31 December 2027.

  3. Check when HMRC's notice was served on your company. If it late, you may have longer. See "When it can be later" below.

  4. Pay your tax first. It is due 9 months and 1 day after your ends.

  5. File online with commercial software. Send the , your accounts and your .

Sources

How it works

The deadline is usually 12 months after your period ends

Your is due 12 months after the end of the it covers. HMRC counts to the same date in the same month of the next year.

The law sets the as the latest of four dates. When your accounts and your accounting period cover the same 12 months, the first two give the same date. The other two only matter if your accounts run for more than 18 months, or HMRC's notice late. So the deadline is usually 12 months.

For example

Bramley Studio Ltd's accounting period ends on 31 December 2026.

Its accounts cover the same year, and HMRC's notice was served in good time.

Its return is due by 31 December 2027.

Sources

The detail The exact rule

Finance Act 1998, Schedule 18, paragraph 14(1): the is the last day of whichever of these periods is the last to end: (a) twelve months from the end of the period for which the return is made; (b) if the company's is not longer than 18 months, twelve months from the end of that period; (c) if the relevant period of account is longer than 18 months, 30 months from its beginning; (d) three months from the date on which the notice requiring the return was served. Paragraph 14(2): the relevant period of account is the period of account in which the last day of the accounting period falls. HMRC's manual (CTM93030) restates the four limbs.

HMRC's manual (CTM93040): for a return period ending on any day other than 29 February, the filing date is the correspondingly numbered day of the corresponding month of the following year. If the period ends on 28 February in the year before a leap year, the filing date is 29 February. If it ends on 29 February, the filing date is 28 February of the following year.

Sources

When it can be later

HMRC's notice was served late. You always have at least 3 months from the day the notice is served on your company. So if it is served less than 3 months before the usual deadline, you get longer. The 3 months start on the day after the notice is served.

HMRC treats a notice sent by second class post as served on the third working day after posting. For first class post, it is the next working day. Your company can prove otherwise.

For example

Say HMRC's notice for Bramley's year to 31 December 2026 is served on 15 November 2027.

Three months from then ends on 15 February 2028. That is later than 31 December 2027.

Bramley's return is due by 15 February 2028.

Your accounts cover more than 12 months. An accounting period cannot be longer than 12 months, so you file 2 returns. If your accounts cover 18 months or less, both returns are due 12 months after your accounts end. That is unless a late notice gives you longer.

That works because the law takes the latest date. For the first return, 12 months after the end of your accounts is later than 12 months after the end of its period.

For example

Say your company's year end was 31 December 2024, and you lengthened it to 31 March 2025.

You file 2 returns.

Both are due by 31 March 2026. You must also update your accounting period dates with HMRC before 31 December 2025.

Sources

The detail The exact rule

Paragraph 14(1)(d) gives three months from the date the notice was served. HMRC's manual (CTM93040): the three months start on the day after the notice was served. If service is on the last day of a month, they end on the last day of the third following month. Otherwise they end on the correspondingly numbered day in the third following month, or its last day if there is no such day. For these purposes the date of service is the third working day after posting for second class post, and one working day after posting for first class post, unless the company proves otherwise (Interpretation Act 1978, section 7). Section 7 itself deems service by post to be effected at the time the letter would be in the ordinary course of post, unless the contrary is proved.

GOV.UK says that if you lengthen your , you must contact HMRC to update your accounting period dates before the original filing date of your return. You may get a late filing penalty if you do not.

Sources

What goes in the return

A Corporation Tax return is made up of:

  • any

  • your company's accounts

  • your

Your accounts and tax calculations must be in a digital format called . You must file online using . HMRC's own online filing service closed on 31 March 2026. You can only file on paper if you have a reasonable excuse or are filing in Welsh.

When you file, you work out your company's profit or loss for Corporation Tax. This is different from the profit or loss in your accounts. The return must include a declaration that it is correct and complete, to the best of the knowledge of the person making it.

Sources

The detail The exact rule

GOV.UK: the computations and accounts must be in Inline eXtensible Business Reporting Language (iXBRL); you must use proprietary third party software; company returns must be filed electronically for any accounting period ending after 31 March 2010, and a company or agent must register with HMRC before filing. The page's "Format" section was updated on 1 April 2026 following the closure of the HMRC online filing service on 31 March 2026. The return must be for the accounting period that is the same as, or ends in, the period given in the notice.

Finance Act 1998, Schedule 18, paragraph 3: an officer may by notice require a company to deliver a return of such information, accounts, statements and reports as the notice reasonably requires. It must include the declaration, and be delivered by the filing date. Paragraph 11: for a company in the UK throughout the period, and required under the Companies Act 2006 to prepare accounts for a period covering the whole of it, HMRC can require only the accounts that Act requires, with the information and documents it requires.

Sources

Your tax is due before the return

Your Corporation Tax is due 9 months and 1 day after your accounting period ends. That is about 3 months before the return.

A company only instead if its are over £1.5 million a year, less with associated companies, and its tax is over £10,000 a year. The law makes the normal date subject to the instalment rules. when to pay Corporation Tax has the full test.

For example

Bramley's accounting period ends on 31 December 2026.

Nine months later is 30 September 2027. The day after is 1 October 2027.

Its tax is due by 1 October 2027, and its return by 31 December 2027.

Sources

Companies House has its own deadline

Your company's accounts also go to Companies House. They are due 9 months after your company's financial year ends. You can file with Companies House and HMRC together or separately.

If your cover more than 12 months, the Companies House deadline is different. They are due 21 months after the company was , or 3 months after the year end if that is later.

Sources

Consequences

If you get it wrong

If your return is late, your company pays a penalty. The later it is, the more it pays.

  • How late

    1 day

    Penalty
    £200

  • How late

    3 months

    Penalty
    Another £200

  • How late

    6 months

    Penalty
    HMRC estimates your tax, and charges 10% of the unpaid tax

  • How late

    12 months

    Penalty
    Another 10% of any unpaid tax

Sources

The law sets the flat penalty at £200 if the return is filed within 3 months of the , and £400 after that. GOV.UK shows the same total as £200, then another £200.

These amounts apply to returns due on or after 1 April 2026. If your return is late 3 times in a row, the £200 penalties rise to £1,000 each.

If you have , you can . File the return first. Then fill in HMRC's appeal form online, print it and post it.

Sources

The detail The exact rule

Finance Act 1998, Schedule 18, paragraph 17: a company that fails to its return by the is liable to a flat-rate penalty of £200 if the return is delivered within three months after the filing date, and £400 "in any other case". The amounts are £1,000 and £2,000 for a third successive failure. The company may also be liable to a tax-related penalty under paragraph 18. Finance Act 2026, section 265 doubled the earlier amounts (£100, £200, £500 and £1,000) for returns with a filing date on or after 1 April 2026. GOV.UK adds that at 6 months late HMRC will write telling you how much tax it thinks you must pay, and you cannot appeal against that estimate.

Sources

Less common situations

Your deadline falls on a weekend or bank holiday. HMRC counts the as a calendar date. A concession that gave extra time ended on 31 March 2022. HMRC's rules now give no extra time for weekends or bank holidays. Returns must be submitted by the . To be safe, file by the working day before.

Your company is dormant. Once you have told HMRC, you do not file another return unless HMRC asks or you start again. See "Does this apply to me?" above.

Sources

Worked example: Bramley Studio Ltd

Bramley Studio Ltd's and its accounts both run from 1 January to 31 December 2026. HMRC's notice more than 3 months before 31 December 2027. Its Corporation Tax for 2026 is £5,225.

Deadlines for the year to 31 December 2026

  • Accounts to Companies House

    Date
    Thursday 30 September 2027

    9 months after the year end

  • Corporation Tax of £5,225

    Date
    Friday 1 October 2027

    The day after 9 months from the period end

  • Corporation Tax return

    Date
    Friday 31 December 2027

    12 months after the period end. Not a bank holiday in England and Wales

Sources

If the return is late

  • Filed on

    15 January 2028

    Penalty
    £200

    Within 3 months of the deadline

  • Filed on

    1 May 2028

    Penalty
    £400

    More than 3 months late

  • Filed on

    Not filed by 30 June 2028

    Penalty
    Also 10% of the unpaid tax

    6 months late. Our working: if all £5,225 is unpaid, 10% is £522.50

Sources

Not covered here

How to work out the tax, amending a return, and penalties for paying late. See when you pay Corporation Tax, HMRC penalties for a late return and the filing calendar for a year ending 31 December 2026.

Where this comes from

58 official sources. Open a group to see them.

The law 18 sources
HMRC 14 sources
GOV.UK 20 sources
Companies House 6 sources

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