What do I need to file a CT600? A filing-day checklist

In short: To file a Company Tax Return (CT600) you need the company's UTR and registration number, the accounting period dates, the accounts and tax computation in iXBRL, figures for any supplementary pages such as the CT600A, and the company's own Government Gateway sign-in. HMRC says a return is the CT600, supplementary pages, accounts and computations (GOV.UK: Company Tax Return obligations). Taxley prepares and files the CT600 for £44.50 (promotion price until 31 Dec 2026; £89.00 from 1 Jan 2027) with micro-entity accounts, then files the accounts at Companies House once the return is paid and finished.

Take the 30-second check to see whether Taxley fits your company, or work out your dates with the Corporation Tax deadline calculator.

Key facts (checked on 4 October 2026)

What do you need before you file a CT600?

You need 12 things: the company's UTR, registration number and period dates, the iXBRL accounts and tax computation, figures for any supplementary pages, losses brought forward, last year's accounts, the declarant's details, the company's Government Gateway sign-in, the Companies House authentication code and the 17-character payment reference. The table shows where to find each one.

The first nine items build the return, the Government Gateway sign-in sends it, and the last two belong to separate jobs. The authentication code files the annual accounts at Companies House, not the CT600 at HMRC (GOV.UK: Company authentication codes), and the payment reference pays the tax, which has its own deadline (GOV.UK: Pay your Corporation Tax bill). Closing the books comes first, as our limited company year-end checklist explains. For the filing steps themselves, see how to file a CT600 yourself.

Which items does HMRC require by law, and which are practical?

By law the return must contain the CT600, any relevant supplementary pages, the company's accounts, and computations showing how the return's figures come from those accounts, with a declaration. The accounts and computations go online in iXBRL. Everything else on the checklist helps you complete, send or pay for that return.

The duty comes from the notice to deliver: the Finance Act 1998 obliges a company to deliver the information, accounts, statements and reports a notice reasonably requires (FA 1998 Sch 18 para 3). HMRC's manual says the notice, form CT603, in practice requires a completed CT600 with any relevant supplementary pages, computations linking the return to the accounts, and a copy of the full accounts, including any directors' and auditors' reports (HMRC manual CTM93090). Returns for periods ending after 31 March 2010 go online, the accounts and computations must be in iXBRL, and the person making the return declares it correct and complete (GOV.UK: Company Tax Return obligations).

Where do you find the company's UTR and registration number?

The UTR is on HMRC's letters to the company, such as the notice to deliver a Company Tax Return: it is the last 10 digits of the 13-digit number at the top (HMRC: Completing your Company Tax Return). The company registration number is on the certificate of incorporation, on letters from Companies House and on the public Companies House register.

HMRC's guide says to enter the registration number only if the company is registered at Companies House (HMRC: Completing your Company Tax Return). A lost company UTR can be requested online, and HMRC posts it to the business address registered with Companies House (GOV.UK: Find your UTR number). The company's UTR is a different number from a director's personal Self Assessment UTR, so check it against an HMRC letter addressed to the company. Our guide to what a UTR is and where to find it covers the search step by step.

Which accounting period does the return cover?

One accounting period of no more than 12 months, normally the same as the financial year in the company's annual accounts. HMRC writes with the first period's dates once Corporation Tax services are added to the business tax account, and you can check later periods by signing in. Accounts longer than 12 months need two returns.

A company that lengthened its financial year must contact HMRC before the original filing date to update its accounting period dates, or it may get a late filing penalty (GOV.UK: Accounting periods for Corporation Tax). The deadlines all run from the period end: the return is due 12 months after it (GOV.UK: Company Tax Returns), the tax 9 months and 1 day after it (GOV.UK: Pay your Corporation Tax bill), and a private company's accounts 9 months after its financial year ends (GOV.UK: Accounts and tax returns for private limited companies). For a 12-month period ending 31 March 2026 that is not the company's first, the accounts are due at Companies House by 31 December 2026, the tax by 1 January 2027 and the return by 31 March 2027. Our Corporation Tax deadlines guide covers first and long periods.

What accounts and tax computation does the return need?

The company's statutory accounts for the period, and a tax computation showing how each figure on the CT600 was worked out from those accounts. HMRC requires both in iXBRL, a tagged format that filing software produces, so a plain PDF of the accounts isn't enough on its own for an online return.

HMRC's guide says to tick box 80 when you send accounts for the return's period, to include computations showing how the return's figures come from the accounts, and to explain in box 90 if you don't (HMRC: Completing your Company Tax Return). HMRC's own "File your accounts and Company Tax Return" service closed on 31 March 2026, and HMRC now says to use commercial software, checking that it files a CT600, a Corporation Tax computation and the company accounts (GOV.UK: Filing company accounts and tax returns). Our guides explain what to check in iXBRL accounts beyond the PDF and how to choose CT600 software now HMRC's service has closed.

Which extra figures might the return need?

Figures for any supplementary pages the company needs, and for losses brought forward. The CT600A, for example, reports loans to participators, such as money a director or shareholder owed the company at the end of the period. Losses from earlier periods need last year's computation, because this year's computation must identify them.

GOV.UK says to use form CT600A to show a director's loan still owed at the end of the accounting period, and if it isn't repaid within 9 months of the period end, the company pays Corporation Tax of 33.75% of the amount outstanding, or 32.5% for a loan made before 6 April 2022 (GOV.UK: Director's loans) and 35.75% for one made from 6 April 2026 (HMRC manual CTM61505). Have the loan account balance at the period end and any repayments since; our CT600A guide goes through the boxes. For losses, HMRC says the computation for periods ending on or after 1 April 2017 must identify unused losses carried forward, split between losses arising before and after that date, and state the deductions allowance used (HMRC: computations).

Why do you need the company's own Government Gateway sign-in?

Because the return goes to HMRC under the company's own Government Gateway user ID and password, the sign-in for its HMRC business tax account with Corporation Tax added. The user ID has up to 12 characters, and GOV.UK says a director can't use their personal Government Gateway ID for the company.

GOV.UK says the company gets its own user ID and password when you register it, separate from any personal one (GOV.UK: Set up a private limited company). Corporation Tax services are then added to the business tax account with an activation code that arrives within 10 days, or 21 days abroad (GOV.UK: Add Corporation Tax services). The sign-in is a user ID, not an email address (GOV.UK: HMRC online services). Our guides cover finding the company's user ID, getting the activation code and a sign-in rejected with error 1046.

Do you need the Companies House authentication code to file a CT600?

Not for HMRC. The CT600 goes to HMRC with the Government Gateway sign-in, while the authentication code belongs to Companies House and is needed to file the company's accounts there online or through software. In a normal year the accounts are due at Companies House before the CT600 is due at HMRC, so have the code ready.

Companies House issues each company a 6-character code, the equivalent of an officer's signature, needed to file online or with commercial software (GOV.UK: Company authentication codes). It is posted to the registered office and can take up to 10 working days to arrive, or it can go to a home address if you can't get post there. The accounts can go to Companies House and the Company Tax Return to HMRC together or separately (GOV.UK: Accounts and tax returns for private limited companies). Our guide on a lost Companies House authentication code explains what to do if the letter has gone missing.

How do you pay the Corporation Tax after filing?

Pay HMRC separately, quoting the company's 17-character Corporation Tax payment reference for that accounting period. GOV.UK says it's on the notice to deliver the return, on HMRC reminders and in the company's HMRC online account, and it changes every period. The tax is usually due 9 months and 1 day after the period ends.

GOV.UK warns that a wrong reference can delay the payment or put it against a different tax bill, and that a bank which saved HMRC as a payee keeps the last reference used (GOV.UK: Pay your Corporation Tax bill). Corporation Tax must be paid electronically, and the payment deadline falls before the filing deadline (GOV.UK: Company Tax Return obligations). A company with a loss or no tax to pay still files the return (GOV.UK: Company Tax Returns). Our guide on how to pay Corporation Tax has HMRC's bank details and payment times.

What does Taxley ask for when you file a CT600?

Taxley asks for each item at the step that uses it: the company and period when you start, the figures and UTR while you prepare, the Government Gateway sign-in only when you file with HMRC, and the Companies House authentication code before filing the accounts there. It prepares the accounts and tax computation in iXBRL itself.

Taxley (taxley.co.uk), UK online software that prepares and files the Company Tax Return (CT600) with HMRC and the annual accounts with Companies House, offers a Companies House look-up by company name or number that fills in the name and registration number. You pick the period from the dates Companies House holds, or type them, and a period can only be filed once it has ended. The UTR goes in the company's details and is required before you can file; Taxley warns you if it fails HMRC's check-digit test. You enter the profit and loss account and balance sheet, and answer whether a director or shareholder owed the company money at the period end; Taxley works out the CT600A from the loan details. Losses brought forward and last year's figures come from the company's Taxley return for the period just before (for losses, once filed); otherwise you enter them.

Taxley builds the accounts and computation in iXBRL, or you can upload your own iXBRL accounts, which it checks against the return but doesn't file at Companies House. You confirm the declaration and pay the fee, £44.50 (promotion price until 31 Dec 2026; £89.00 from 1 Jan 2027) with micro-entity accounts or £84.50 (promotion price until 31 Dec 2026; £169.00 from 1 Jan 2027) with small-company accounts, which also covers filing the accounts at Companies House. Then you enter the company's own Government Gateway user ID and password to file with HMRC; Taxley never keeps the password. You save the authentication code once in the company's details, and Taxley files the accounts at Companies House once the return is paid and the accounts are finished. Taxley shows the tax to pay and its due date, but doesn't pay it or work out the payment reference. It doesn't file the confirmation statement and isn't tax advice, so review your figures before you file. Take the 30-second check to see whether Taxley fits your company, or see the price for your company.

Frequently asked questions

Can I file the CT600 before I pay the Corporation Tax?

Yes. Filing and paying are separate jobs with separate deadlines: the tax is usually due 9 months and 1 day after the accounting period ends, and the return 12 months after it ends (GOV.UK: Company Tax Returns). Filing early doesn't move the payment date.

Do I need last year's CT600 to file this year's return?

Not by law, but it helps. This year's computation must identify unused losses carried forward from earlier periods (HMRC: Completing your Company Tax Return), and last year's computation shows them. Last year's accounts also supply the comparative figures.

What if I can't find the notice to deliver a Company Tax Return?

The details on it are available elsewhere. The UTR is on other HMRC letters, or you can request it online (GOV.UK: Find your UTR number). The payment reference is in the company's HMRC online account under the Corporation Tax statement (GOV.UK: Pay your Corporation Tax bill).

What is the penalty for filing a CT600 late?

For returns due from 1 April 2026, £200 at 1 day late and £200 more at 3 months (GOV.UK: increases), then 10% of the tax still unpaid if the return is 6 months late (18 months after the accounting period ends), rising to 20% if it is 12 months late (2 years after the period ends) (GOV.UK: Company Tax Returns).

General information, not personalised tax or accounting advice.