The CT600 company tax return explained

A Company Tax Return is more than the CT600 form. It is the CT600, any supplementary pages, the company’s full statutory accounts and tax computations showing how the return figures were reached from those accounts, filed online with the accounts and computations in iXBRL. Each return covers one accounting period of no more than 12 months and is normally due 12 months after that period ends, while the tax is normally due nine months and one day after the period ends.

When does a company have to file a CT600?

The duty comes from a notice. Paragraph 3 of Schedule 18 to the Finance Act 1998 lets HMRC require a company to deliver a return of the information, accounts, statements and reports relevant to its tax liability, and HMRC’s Company Taxation Manual at CTM93090 explains that in practice the notice is form CT603. GOV.UK says a company that receives the notice must file even if it makes a loss or has no tax to pay. A company that is chargeable but has received no notice must tell HMRC within twelve months of the end of the accounting period (paragraph 2 of Schedule 18), as covered in registering for corporation tax. A return sent without a notice is treated as if a notice had been served on the day it was delivered (CTM93050, example 5).

The notice specifies a period, which may not match the company’s accounting periods. CTM93020 explains that a return is needed for each accounting period ending in the specified period, so a single notice can require two returns.

What does the main form contain?

The current form is the CT600 (2015) version 3. HMRC’s guide, Completing your Company Tax Return, walks through it box by box. In outline:

Part	What it covers
Boxes 1 to 4	Company name, registration number, UTR and company type, such as close investment holding company, property management company or non resident company
Boxes 30 to 65	Start and end of the return period, repayment claims, claims affecting earlier periods, more than one return being filed, estimated figures, membership of a group that is not small, and disclosable avoidance schemes
Boxes 70 and 75	Transfer pricing compensating adjustments and the small and medium sized enterprise exemption
Boxes 80 to 90	Whether accounts and computations are attached, and an explanation if not
Box 145 onwards	Turnover, trading profits, interest and other non trading loan relationship profits, property income, chargeable gains, losses, qualifying charitable donations and group relief
Tax calculation	Corporation tax chargeable, marginal relief, double taxation relief, tax on loans to participators, and credits such as research and development expenditure credits
Boxes 920 to 940	Bank details for any repayment

Box 55 is worth noting: HMRC asks for an X if estimated figures have been used. The return figures feed into the rate calculation, including marginal relief where profits sit between the lower and upper limits.

Which supplementary pages might be needed?

HMRC’s guide says supplementary pages form part of the return and are covered by the declaration. The current pages published alongside the CT600 are:

Page	Subject
CT600A	Close company loans and arrangements to confer benefits on participators, often a director’s loan account
CT600B	Controlled foreign companies, foreign permanent establishment exemptions, hybrid and other mismatches
CT600C	Group and consortium relief
CT600D	Insurance
CT600E	Charities and community amateur sports clubs
CT600F	Tonnage tax
CT600H	Cross border royalties
CT600I	Supplementary charge on ring fence trades
CT600J	Disclosure of tax avoidance schemes
CT600K	Restitution tax
CT600L	Research and development
CT600M	Freeports and investment zones
CT600N	Residential property developer tax
CT600P	Creative industries reliefs

Most small trading companies need none, or only CT600A. The relevant box on the main form must be ticked for each page included, and several CT600 boxes take their figures straight from a supplementary page, for example the loans to participators tax from box A80.

What accounts and computations go with it?

CTM93090 says the CT603 notice requires a completed CT600 and relevant supplementary pages, computations showing how the entries were derived from the accounts, and a copy of the full accounts including the directors’ and auditors’ reports. CTM93180 adds a point many miss: HMRC wants the full accounts prepared for members, not the filleted version a small company may file at Companies House with the profit and loss account removed. A holding company must send its own individual accounts as well as group accounts.

The computations reconcile the accounts profit to taxable profit, adding back disallowable items, deducting capital allowances and showing losses. For periods ending on or after 1 April 2017, HMRC’s guide says they must identify losses brought forward split between those arising before and after 1 April 2017, how they were used, and the deductions allowance used. The guidance on small company filing options explains the Companies House side.

How is the return filed?

HMRC’s Company Tax Return obligations guidance says returns must be filed electronically for accounting periods ending after 31 March 2010, with the computations and accounts in Inline XBRL. HMRC’s own free filing service closed on 31 March 2026, so from 1 April 2026 companies must use commercial software. A paper CT600 is only accepted where there is a reasonable excuse or the company is filing in Welsh.

The return includes a declaration that it is correct and complete to the best of the signatory’s knowledge. Where an agent prepares and submits it, the agent makes the declaration. Where the agent only submits it, the proper officer or another authorised person must make it, and the agent must first keep a copy and get that person’s confirmation.

When is it due, and what happens if it is late?

Paragraph 14 of Schedule 18 sets the filing date as the last of: 12 months from the end of the return period; 12 months from the end of the period of account if that is not over 18 months; 30 months from the start of the period of account if it is longer; or three months from the date the notice was served. The tax itself is usually due nine months and one day after the accounting period ends; see corporation tax deadlines.

Late filing penalties under paragraphs 17 and 18 are:

• £200 if the return is up to three months late, and £400 after that, for returns with a filing date on or after 1 April 2026 (Finance Act 2026, section 265);
• £1,000 and £2,000 instead for a third successive late return;
• a tax related penalty of 10% of the tax still unpaid if the return has not been delivered within 18 months after the end of the period, rising to 20% if it is delivered more than two years after the period ends.

GOV.UK adds that once a return is six months late, HMRC issues a determination of the tax it thinks is due, which cannot be appealed; it is displaced by filing the return.

Can it be changed, and when can HMRC check it?

GOV.UK says amendments must usually be made within 12 months of the filing deadline, using software or by writing to HMRC; after that, overpayment relief may be available (see amending a tax return). Under paragraph 24 of Schedule 18, HMRC can open an enquiry into a return filed on time within 12 months of the day it was delivered, or 12 months from the filing date for a member of a group that is not small. Late or amended returns stay open longer, to the quarter day following the first anniversary of delivery or amendment.

What does a typical timetable look like?

Illustration only. A company draws up accounts for the year to 30 June 2026. Its corporation tax is due on 1 April 2027 and its CT600, accounts and computations by 30 June 2027. If it is a standalone company and files on 10 February 2027, HMRC has until 10 February 2028 to open an enquiry. Had the accounts run for 15 months, there would be two returns and two payment dates, but both returns would still be due 12 months after the end of the accounts period.

What are the common mistakes?

• Attaching filleted accounts rather than the full accounts prepared for members.
• Computations that do not reconcile to the accounts, or that omit the loss analysis HMRC requires.
• Forgetting CT600A when a director’s loan account is overdrawn at the year end.
• Entering a return period longer than 12 months; long accounts need two returns.
• Treating the filing deadline as the payment deadline, and paying three months late.

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Related Accountapedia entries

• Your company’s first corporation tax return
• What is a corporation tax computation
• iXBRL tagging for company tax returns
• How to amend a company tax return
• Long first accounts: splitting into two tax returns
• Does an accounts extension extend the CT600 deadline
• Which CT600 supplementary pages a company needs
• Filing a CT600 after HMRC’s free filing service closed
• CT603 notice to deliver a company tax return

Sources

• legislation.gov.uk: Act 1998 c. 36, schedule 18
• GOV.UK: Manual page CTM93090
• GOV.UK: Company tax returns
• legislation.gov.uk: Act 1998 c. 36, schedule 18
• GOV.UK: Manual page CTM93050
• GOV.UK: Manual page CTM93020
• GOV.UK: Corporation tax company tax return ct600 2015 version 3
• GOV.UK: The company tax return guide
• GOV.UK: Corporation tax forms
• GOV.UK: Manual page CTM93180

Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 8 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.