Company tax return (CT600): who files, what it contains and how to file it

What is a company tax return?

A Company Tax Return is the annual return a limited company sends to HMRC to report its Corporation Tax position for an accounting period. The main form is the CT600. When you file it, you work out the company's profit or loss for Corporation Tax and its tax bill. GOV.UK notes that the Corporation Tax profit or loss is different from the profit or loss in the annual accounts.

People often search for "company tax return", "CT600" and "Corporation Tax return" as if they were three things. They are one thing. The return is the CT600 plus its attachments. Source: GOV.UK: Company Tax Returns.

Who has to file a Company Tax Return?

A company or association must file when it receives a "notice to deliver a Company Tax Return" from HMRC. You must still send a return if the company made a loss or has no Corporation Tax to pay.

Sole traders and partners do not send a Company Tax Return. They send a Self Assessment return instead. A director who also has untaxed income, such as dividends, files a personal return as well. The two are separate filings. See Self Assessment for directors and what Self Assessment is.

HMRC does not post a Corporation Tax bill. The company works out its own figures. You will need the company's 10-digit UTR to file, so keep it safe. Our guide to the company UTR number covers where to find it.

What does the return contain?

A complete Company Tax Return has these parts:

• The CT600. A 12-page form with the company details, the tax calculation, claims, any tax due or repayable and the declaration.
• Company accounts. You tick box 80 on the form to show you are submitting accounts for the period the return covers.
• Tax computations. These must show how the figures in the return were calculated from the figures in the accounts. GOV.UK asks for them in a single iXBRL computations file.
• Supplementary pages, only where the company's facts call for them.

If the company does not submit accounts and computations, box 90 is where you explain why. The line-by-line instructions are in HMRC's Completing your Company Tax Return.

The return is a signed declaration. HMRC's acknowledgement only confirms receipt. It does not mean HMRC agrees with the figures.

Which supplementary pages might apply?

Supplementary pages sit alongside the CT600, and you tick the matching boxes on the form. They form part of the return and are covered by the declaration. Warnings about late or incorrect returns apply to them as well. HMRC lists these pages with the CT600 form:

There are others, for example for insurance, Tonnage Tax, Freeports and Residential Property Developer Tax. Most small trading companies use none of them. A company that borrowed from, or lent to, its owners during the year is the usual exception. Read our guide to allowable deductions to see what feeds the computation.

How do you file a company tax return in 2026?

The HMRC online service "File your accounts and Company Tax Return" closed on 31 March 2026. From 1 April 2026, you file using commercial software. The free HMRC online service is no longer available.

The software must be able to produce what you need: a CT600, a Corporation Tax computation and your company accounts. HMRC keeps a list of recognised commercial software suppliers, and some products file the return and the accounts together. Companies House accounts are filed separately, and Companies House has its own software list.

You can only file a paper return if you have a reasonable excuse for not filing online, or you are filing in Welsh. The paper CT600 is also the form for amending a return you have already submitted. If an IT problem with HMRC's online services stops you filing, HMRC says to send form WT1 with the paper return.

You can prepare and file the return yourself or appoint an accountant to do it. Our Corporation Tax service covers the second route.

Filing deadline versus payment deadline

The return is due 12 months after the end of the accounting period it covers. The Corporation Tax is usually due earlier: 9 months and 1 day after the period ends, if taxable profits are up to £1.5 million. Above that, GOV.UK says the tax is paid in instalments, with its own rules and dates.

The accounting period is usually the financial year, but can differ, particularly in the first year. It cannot be longer than 12 months, so a company whose accounts cover a longer span files two returns. The Companies House accounts deadline is a third, separate date. See the Corporation Tax deadlines guide and financial year vs tax year. You cannot pay Corporation Tax by post, so allow time for the payment method you pick: see GOV.UK: pay your Corporation Tax bill.

Penalties for late filing

If the return is late three times in a row, each £200 penalty rises to £1,000. After 6 months, HMRC writes with a "tax determination" of the tax it thinks is due. You cannot appeal against it. You must pay the tax and file the return, and HMRC then recalculates interest and penalties.

With a reasonable excuse, you can appeal a late filing penalty online, but you must file the return first. You will need the company UTR, the date on the penalty notice, the amount and the end date of the accounting period. Pay penalties using the 17-character Corporation Tax payment reference for that period. Our late filing penalties guide goes further.

Nil returns, losses and dormant companies

There is no exemption for a quiet year. A company with a loss, or nothing to pay, still files if HMRC has issued a notice to deliver. Late filing penalties apply even when no tax is due.

A dormant company is different, but only after you act. You can tell HMRC your company is dormant for Corporation Tax. After that, you do not have to file another Company Tax Return unless HMRC asks or the company starts trading again. You must still send information to Companies House every year. Charities, community amateur sports clubs and flat management companies that are dormant may also be able to apply for a filing exemption.

Amending a Company Tax Return

You must usually make changes within 12 months of the filing deadline. You can use commercial software, or send a paper return or a letter to the Corporation Tax office. HMRC may check a return for errors and may charge a penalty for them.

After that window, the route depends on the direction of the error. If the company overpaid, it may still claim the tax back through overpayment relief. If it underpaid, GOV.UK says to tell HMRC through the online disclosure service as soon as possible. See GOV.UK: making changes.

Records and common errors

• Treating the Companies House accounts, the Corporation Tax payment and the CT600 as one deadline.
• Filing from incomplete bank, payroll, sales or director-loan records.
• Treating a capital or personal cost as a normal trading expense.
• Claiming a relief without the records that support it.
• Leaving it too late to choose software or an agent after the 31 March 2026 closure.

GOV.UK says companies normally keep records for six years from the end of the financial year they relate to, and longer in some situations. Keep the accounts, computations, invoices, bank records, payroll information and papers behind each relief together, so an enquiry can be answered quickly.