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. You file it within 12 months of your accounting period end (and pay any tax due earlier — by 9 months and 1 day).

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?

When HMRC says "file your Company Tax Return," they mean three things together:

The accounts and computations must be attached in iXBRL — inline eXtensible Business Reporting Language — which is a normal-looking document with machine-readable tags behind each number so HMRC's systems can read it automatically.

What do you need to file?

Before you can submit, you'll need:

• 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?

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 through the Corporation Tax Online service (software sends it via HMRC's secure gateway).
5. Get HMRC's response — accepted, or rejected with errors to correct.

You can't file a Company Tax Return on paper except in narrow circumstances — it's an online process.

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.

What is iXBRL and why does HMRC need it?

iXBRL is a tagged version of your accounts and computations that HMRC's systems can read automatically. HMRC requires Company Tax Return attachments in this format.

Can I file the CT600 myself without an accountant?

For a straightforward company, yes — software can prepare and submit it. But you remain responsible for the accuracy of the figures, and anything beyond a simple case (capital allowances, loss relief, groups) is worth having a qualified adviser check.

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.