What is a CT600 and how do you file one?

In short: A CT600 is the HMRC form that reports your company's profit and works out its Corporation Tax. You don't file it on its own — a full Company Tax Return is the CT600 plus your statutory accounts and tax computations, submitted to HMRC online in a tagged format called iXBRL (GOV.UK: Company Tax Returns: Overview). You file it within 12 months of your accounting period end (and pay any tax due earlier — by 9 months and 1 day).

You can prepare a CT600 with Taxley from your own figures — it builds the form, the iXBRL accounts and the computations together, ready to file. Not sure it suits your company? Take the 30-second check to see whether Taxley fits.

What is a CT600?

The CT600 is the Company Tax Return form for UK Corporation Tax. It's a structured set of numbered boxes that tell HMRC your company's income, deductions, profit chargeable to tax, and the Corporation Tax due. Every box maps to a specific figure — for example, trading profit, property income, the tax rates applied, and any reliefs.

By itself the CT600 is just the summary form. HMRC wants the supporting detail too.

What makes up a full Company Tax Return?

A full Company Tax Return is three things together: the CT600 form, your statutory accounts and your tax computations. The accounts and computations are attached in iXBRL, and the whole return goes to HMRC online within 12 months of the end of the accounting period.

iXBRL — inline eXtensible Business Reporting Language — is a normal-looking document with machine-readable tags behind each number so HMRC's systems can read it automatically. Our Corporation Tax glossary explains this and other filing terms in plain English.

What do you need to file?

You need four things before you can submit a CT600: a reference from HMRC, a login that can file for the company, a number from Companies House, and the period's figures. You also need commercial software or an agent to send them, because HMRC's own online filing service closed on 31 March 2026. The four things:

• Your company's Unique Taxpayer Reference (UTR) — the 10-digit number HMRC issued for Corporation Tax.
• Government Gateway credentials that are enrolled for Corporation Tax Online.
• Your company registration number (CRN) from Companies House.
• Your figures for the period: income, allowable expenses, and a balance sheet.

How does filing actually work?

You prepare your figures, generate the CT600 with iXBRL accounts and computations, and submit them together to HMRC, which returns an acceptance or a rejection with a reason. Filing is complete only when HMRC accepts the return, and any tax is due separately, 9 months and 1 day after the period ends. In more detail: For a step-by-step walk-through, see how to file a CT600 yourself.

1. Prepare the figures — income and expenses for the accounting period, and a balance sheet that balances.
2. Produce the accounts and computations as iXBRL.
3. Complete the CT600 boxes from those figures.
4. Submit online to HMRC — your filing software sends it through HMRC's secure gateway using your Government Gateway credentials.
5. Get HMRC's response — accepted, or rejected with errors to correct.

HMRC's own online filing service closed on 31 March 2026, so from 1 April 2026 HMRC says you should use commercial software (or an agent). A paper return is accepted only if you have a reasonable excuse or are filing in Welsh (GOV.UK: Closure of HMRC's file-your-accounts-and-tax-return service). Our guide to choosing CT600 software now HMRC's service has closed covers what to check. To see each stage as Taxley handles it, read How to submit your Corporation Tax return online, step by step.

Do you have to file if you made no profit?

Usually, yes: if HMRC has sent your company a "notice to deliver a Company Tax Return," you must file one even if you made a loss or owe no tax. Filing a return and paying tax are separate obligations — and a loss-making year still often needs a return so the loss is recorded.

Frequently asked questions

Is the CT600 the same as my company accounts?

No. The CT600 is the tax return form; your accounts are a separate document filed alongside it (and also, separately, at Companies House). A full submission includes both plus the tax computations.

What's the deadline to file a CT600?

Twelve months after the end of your accounting period. Note the Corporation Tax payment is due earlier — 9 months and 1 day after the period ends. See our Corporation Tax deadlines guide for the full timeline and penalties.

What is iXBRL and why does HMRC need it?

iXBRL is a tagged version of your accounts and computations: a normal-looking document with machine-readable tags behind each number, so HMRC's systems can read it automatically. HMRC requires Company Tax Return attachments in this format, which is why a plain PDF of the accounts isn't enough.

Can I file the CT600 myself without an accountant?

For a straightforward company, yes — software can prepare and submit it (see what it costs on our pricing page). But you remain responsible for the accuracy of the figures, and anything beyond a simple case — such as a terminal loss claim or an R&D claim — Taxley can't file yet.

Taxley prepares the CT600, FRS 105 / FRS 102 §1A accounts and tax computations as iXBRL from the figures you enter, ready to file. You're responsible for the accuracy of the return, and a qualified review is recommended before you file.