The CT600, from first notice to filed return.
A CT600 is the form at the centre of a Company Tax Return. These guides cover what it is, what goes with it, when it’s due and what to do when something goes wrong. Each one is checked against GOV.UK and dated.
Looking for something to file with? See CT600 software, or if you used HMRC’s free service, how to file now it has closed.
What is a CT600?
A CT600 is the form at the centre of a Company Tax Return. Here’s what it is, who has to send one, what’s on it, when it’s due and how it’s filed.
How to file a CT600 (Company Tax Return)
How to file your company’s CT600 online: who must file, what the return includes, what you need, the deadlines, and how to correct a return after filing.
What needs to go with a CT600?
A CT600 doesn’t go to HMRC on its own. Here’s what has to accompany it: the company’s accounts, a tax computation in iXBRL and any supplementary pages.
Corporation Tax deadlines: filing, paying and penalties
When Corporation Tax is due: pay 9 months and 1 day after your accounting period ends, file your CT600 within 12 months. Dates, examples and penalties.
Accounting periods for Corporation Tax explained
A Corporation Tax accounting period can’t be longer than 12 months, so it isn’t always the same as the period of your accounts. Here’s when the two differ and how many returns you file.
What is iXBRL, and does your company need it?
iXBRL is the tagged format HMRC requires for company accounts and tax computations, and Companies House will require for all accounts from April 2028. A plain explanation.
CT600 supplementary pages explained
The CT600 has supplementary pages for particular situations, from CT600A for loans to directors to CT600P for creative industries. Here’s what each is for and which small companies need.
CT600A explained: loans to directors and shareholders
CT600A is the supplementary page for a close company’s loans to its participators. Here’s who completes it, the section 455 tax rates, how relief works when a loan is repaid, and the 2026 rate change.
CT600L explained: the R&D supplementary page
CT600L is the supplementary page for research and development claims on a Company Tax Return. Here’s who completes it, the two forms HMRC needs first, and how it feeds the CT600.
Micro-entity accounts and Corporation Tax
Micro-entities can file very short accounts at Companies House, but their Company Tax Return is the same as any other company’s. Here’s what a micro-entity sends to HMRC, and when.
How to amend a CT600 after filing
You can usually amend a Company Tax Return within 12 months of the filing deadline. Here’s how to do it, what HMRC can correct itself, and what to do once the deadline has passed.
HMRC’s free Company Tax Return service has closed: what to do now
HMRC’s free online service for filing company accounts and Company Tax Returns closed on 31 March 2026. Here’s how to file your CT600 now, and what changes for your Companies House accounts.
CT600 software: what to look for
Now that HMRC’s free filing service has closed, every Company Tax Return needs commercial software. Here’s what to check before you choose, and which kind of product suits which company.
What goes in a Corporation Tax computation?
A Corporation Tax computation takes the profit in your company’s accounts and works out the taxable profit and the tax. Here’s what goes in one, with a worked example.
File your accounts and Company Tax Return from one set of books
A4O Accounts prepares your accounts and CT600 from the same figures, tags them in iXBRL and files them with Companies House and HMRC. Free to get started, no card required.
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