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A DIRECTOR’S FILING GUIDE

CT600 guide: filing your company's Corporation Tax return

Filing for the first time? Work from your accounting records through the tax computation, CT600 Version 3 and submission deadlines—without confusing the company's return with your personal tax return.

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01 / THE BASICS

What is a CT600?

CT600 Version 3 is the Company Tax Return a UK company files with HMRC. An online return normally includes statutory accounts and a tax computation in iXBRL format. For a broader introduction, read the Company Tax Return guide.

02 / FILING OBLIGATIONS

Who must file?

UK limited companies and some other bodies, including clubs, associations and CASCs, must file when HMRC sends a notice to deliver a Company Tax Return. A dormant company must still respond to a notice unless HMRC confirms that it does not need to file.

A notice to deliver a Company Tax Return (CT603) triggers the filing obligation. Do not assume inactivity cancels the notice.

03 / TWO DIFFERENT RETURNS

CT600 vs Self Assessment

CT600 and SA100 Self Assessment compared
QuestionCT600SA100 Self Assessment
Who files?The company.Individuals, including many directors.
TaxCorporation Tax on company profits.Income Tax and, where applicable, Capital Gains Tax on personal income and gains.
Filing deadlineNormally 12 months after the accounting period ends.Normally 31 January after the tax year ends when filing online.
PaymentNormally 9 months and 1 day after period end for most companies.Normally 31 January, with payments on account where applicable.
Accounts attached?Statutory accounts and a tax computation in iXBRL, normally.Not required.

A director may also need to file a personal Self Assessment return; the company's CT600 does not replace it.

04 / THREE DATES TO TRACK

Deadlines

12 monthsCT600 filing deadline after the Corporation Tax accounting period ends.
9 months + 1 dayCorporation Tax payment deadline after period end for most companies outside the instalment-payment regime.
9 monthsPrivate company accounts deadline after the Companies House accounting reference date; first accounts are normally due 21 months after incorporation.

See the Corporation Tax deadlines and penalties guide for more detail.

05 / IF YOU FILE LATE

Penalties

For returns with a filing date on or after 1 April 2026, HMRC's fixed penalty is £200 when the return is one day late, plus a further £200 when it is more than three months late (£400 in total). For a third successive late return, these become £1,000 plus a further £1,000 (£2,000 in total). Tax-geared penalties of 10% of unpaid tax may also apply at 18 and 24 months. Interest may be charged on late payment.

Companies House accounts have a separate penalty regime; see the Companies House late-filing penalties guide.

Sources: GOV.UK penalty increase and GOV.UK late-return rules.

06 / ACCOUNTS STANDARDS

FRS 105 or FRS 102?

Common accounts reporting choices
StandardWho may use itApproach
FRS 105 micro-entityEligible micro-entities; size thresholds and other restrictions apply.Simplified reporting; no fair value accounting.
FRS 102 Section 1A / full FRS 102Eligible small entities may use Section 1A; others may need full FRS 102.Requirements and disclosures depend on the entity and reporting regime.

The tax computation follows the same tax rules either way. Compare micro-entity accounts and small company accounts.

07 / TAXABLE PROFIT

From P&L to taxable profit

Accounting profit is a starting point, not necessarily the taxable amount:

  1. Start with accounting profit from the profit and loss account.
  2. Add back disallowable expenses, such as depreciation, client entertaining and fines.
  3. Deduct applicable capital allowances.
  4. Deduct available losses and other reliefs.
  5. Calculate taxable profit.
  6. Apply the 19% small profits rate or 25% main rate, with Marginal Relief where applicable between £50,000 and £250,000. These limits are divided by the number of associated companies plus one.

Read more about allowable expenses for limited companies.

Let WeFile handle the calculation

Prepare supported accounts, review your Corporation Tax computation and submit your return with a guided workflow. WeFile appears on HMRC's Corporation Tax commercial software suppliers list.

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08 / THE PROCESS

How to file, step by step

HMRC's free CT online filing service closed on 31 March 2026. See the guide to the closure.

  1. Confirm the accounting period and UTR

    Check your company's UTR and the Corporation Tax period held by HMRC.

  2. Finalise bookkeeping and the trial balance

    Reconcile your records before preparing accounts and tax figures.

  3. Prepare statutory accounts

    Prepare accounts under the applicable accounting standard.

  4. Prepare the tax computation

    Adjust accounting profit for tax and calculate the Corporation Tax due.

  5. Complete the CT600 and any supplementary pages

    Review CT600 Version 3 and include the pages relevant to your company.

    Check whether you need CT600A or CT600E.

  6. Submit online

    Use commercial software and your Corporation Tax-enrolled Government Gateway credentials. Keep the electronic receipt and check the subsequent status.

    Learn what HMRC receives, how the IRmark receipt works and why a return may still be rejected in the CT600 submission to HMRC guide.

  7. Pay Corporation Tax

    Pay by the separate payment deadline, even if the return's filing deadline is later.

This guide is general information, not professional accounting or tax advice. Seek qualified advice for complex circumstances.

BEFORE SUBMITTING

Common errors to check

  • Accounting period dates or company identifiers do not match.
  • The Government Gateway account is not enrolled for Corporation Tax.
  • Figures, supporting accounts or required supplementary pages are missing.

See the guide to HMRC CT600 submission errors for further troubleshooting. Submission does not guarantee acceptance; check each authority's response.

QUESTIONS, ANSWERED

CT600 frequently asked questions

A Company Tax Return is normally due 12 months after the end of its Corporation Tax accounting period. Each CT600 can cover no more than 12 months. The deadline for Companies House accounts is separate.
For returns with a filing date on or after 1 April 2026, the fixed penalty is £200 when the return is one day late and a further £200 when it is more than three months late. For three successive late returns, those amounts rise to £1,000 and a further £1,000 respectively. Tax-geared penalties may also apply at 18 and 24 months.
For most companies outside the instalment-payment regime, Corporation Tax is due nine months and one day after the accounting period ends. Larger companies may need to pay quarterly instalments. Payment and CT600 filing have different deadlines.
A dormant company needs to file a CT600 if HMRC has issued a notice to deliver a Company Tax Return and has not withdrawn it. Dormant accounts for Companies House are a separate obligation.
No. A CT600 reports a company's Corporation Tax position. Self Assessment (SA100) is an individual's return for personal income and gains. A director may have both obligations.
No. The CT600 Version 3 form remains the same. The applicable accounting standard changes the presentation and treatment of items in the accounts attached to the return; the Corporation Tax computation begins with accounting profit.
Yes. A director can use commercial filing software to prepare and file the company return, but remains responsible for its accuracy and deadline. Consider professional advice for complex affairs. WeFile appears on HMRC's Corporation Tax commercial software suppliers list; this is not an HMRC endorsement.

This guide and checklist are general information, not professional accounting or tax advice. Directors remain responsible for accurate, complete and timely filings. Seek qualified advice if your circumstances are complex.