What Is a CT600?
Every UK limited company must file a company tax return with HMRC once a year. This form — officially called the CT600 — is how your company reports its taxable profits, claims reliefs and allowances, and calculates how much corporation tax it owes.
Whether your company made a profit, a loss, or no money at all during the year, HMRC still requires you to file. Understanding what the CT600 is, what goes inside it, and when it must be submitted can save you from unnecessary penalties and make the whole process far less daunting.
The CT600 Explained
The CT600 is a standardised form issued by HMRC. "CT" stands for corporation tax, and "600" is the form number. It is the main way UK limited companies report their taxable profits and compute their corporation tax liability for a given accounting period.
Think of it as the business equivalent of a personal Self Assessment tax return — but instead of salary and savings income, it covers your company's trading income, capital gains, expenses, and any special reliefs such as research and development (R&D) credits or capital allowances.
A full company tax return submission to HMRC consists of three documents filed together:
- The CT600 form — the core tax computation and return
- Statutory accounts — your company's profit and loss account and balance sheet, formatted in iXBRL
- Tax computation — a detailed supporting document showing how taxable profit was calculated
Who Needs to File a CT600?
If your company is a UK limited company or unincorporated association and HMRC has sent you a "notice to deliver a Company Tax Return," you must file a CT600.
This applies even if:
- Your company made a trading loss during the accounting period
- Your company is dormant (no activity, no income, no expenses)
- You have no corporation tax to pay because your profits fall below the threshold or are eliminated by reliefs
What the CT600 Contains
The CT600 is laid out as a series of numbered boxes. Each captures a specific figure or piece of information about your company's tax position. Some of the most important include:
Company and period details (Boxes 1–15) Your company name, registered number, Unique Taxpayer Reference (UTR), and the start and end dates of the accounting period being reported.
Income and profits (Boxes 155–235) Trading profits and losses, property income, capital gains, and other income sources. This section builds up your total taxable income before deductions.
Profits chargeable to corporation tax (Box 400) The key figure that determines your tax bill — after all deductions and reliefs have been applied. See our guide to CT600 Box 400: Profits Chargeable for a full breakdown of how this figure is arrived at.
Corporation tax charged (Boxes 430–450) The actual tax charged, at either the main rate or the small profits rate. If your profits fall between £50,000 and £250,000, marginal relief may reduce your effective rate. For the tax charge calculation in full, see CT600 Box 430: Corporation Tax at Main Rate. For how relief is applied to mid-range profits, see CT600 Box 450: Marginal Relief.
Reliefs and credits (Boxes 530–670) R&D tax credits, creative industry credits, capital allowances, and other reliefs your company may be entitled to claim.
Losses (Box 800) Trading losses being carried forward, carried back, or surrendered as group relief — see CT600 Box 800: Losses Summary for details on how losses are reported.
Current Corporation Tax Rates
The rate of corporation tax your company pays depends on its level of taxable profits for the accounting period. The following rates have applied since 1 April 2023:
| Profits level | Rate |
|---|---|
| Up to £50,000 | 19% (Small Profits Rate) |
| £50,001 – £250,000 | Between 19% and 25% (Marginal Relief applies) |
| Over £250,000 | 25% (Main Rate) |
From 1 April 2015 to 31 March 2023, a single rate of 19% applied to all companies regardless of their profit level.
Source: GOV.UK — Corporation Tax rates
CT600 Filing Deadlines
The CT600 must be submitted within 12 months of the end of your company's accounting period.
Corporation tax itself is due earlier — 9 months and one day after the end of the accounting period. This means most companies must pay their tax bill before they actually file the return.
Example timeline:
- Accounting period ends: 31 March 2025
- Corporation tax payment due: 1 January 2026
- CT600 filing deadline: 31 March 2026
Penalties for Filing Late
HMRC applies automatic penalties if you miss the CT600 filing deadline:
| How late | Penalty |
|---|---|
| 1 day late | £200 |
| 3 months late | Additional £200 (£400 total fixed penalties) |
| 6 months late | 10% of the unpaid corporation tax |
| 12 months late | Further 10% of unpaid corporation tax |
Source: GOV.UK — Company tax returns
How to File a CT600
The CT600 is filed online through HMRC-approved corporation tax software. To complete a submission you will need:
- Your company's UTR — a 10-digit number assigned by HMRC when your company was registered for corporation tax
- Your company's statutory accounts in iXBRL format
- A tax computation document
- Details of any reliefs and allowances being claimed
If your accounting period is longer than 12 months — which can happen in your company's first year — HMRC requires you to file two separate CT600 returns: one covering the first 12 months and one covering the remainder.
Summary
The CT600 is the company tax return form that every UK limited company must file with HMRC. It reports taxable profits, claims reliefs and allowances, and calculates the corporation tax due for the accounting period. Filing is required even when no tax is owed. The return must be submitted within 12 months of the accounting period end — though corporation tax is paid earlier, within 9 months and one day. Missing the deadline triggers automatic penalties starting at £200, rising steeply if the return remains outstanding.