Checked against GOV.UK and Companies House guidance · Updated 6 November 2026
What is the CT600, and who has to file it?
The CT600 is HMRC's Company Tax Return. A company files one for each accounting period when HMRC has sent it a "notice to deliver a Company Tax Return", which almost every trading company gets. According to GOV.UK's Company Tax Returns guide, the return must be filed online, with:
- the CT600 form itself;
- the company's statutory accounts, in iXBRL;
- the tax computation showing how taxable profit was worked out, also in iXBRL.
A dormant company may not need to file at all. The guide What is a dormant company? explains when HMRC stops expecting returns.
How much corporation tax will the company pay?
For the financial year from 1 April 2026, the rates are unchanged from the three years before, according to HMRC's rates and allowances (updated 1 April 2026):
| Taxable profits | Rate |
|---|---|
| Up to £50,000 | 19% (small profits rate) |
| £50,001 to £250,000 | 25%, reduced by marginal relief (fraction 3/200) |
| Over £250,000 | 25% (main rate) |
The £50,000 and £250,000 limits are divided between associated companies and cut for periods shorter than 12 months. Between the limits, each extra pound of profit is taxed at an effective 26.5%. The corporation tax calculator shows the working for any profit figure.
When are the CT600 and the tax due?
| What | When |
|---|---|
| Pay corporation tax (profits up to £1.5m) | 9 months and 1 day after the period ends |
| File the CT600 | 12 months after the period ends |
| Accounts to Companies House | 9 months after the accounting reference date |
The payment deadline comes before the filing deadline, which catches people out. FileMyLtd sends reminders for both, and the corporation tax deadlines guide works through examples.
What happens if the CT600 is late?
HMRC's late filing penalties doubled for returns with a filing date on or after 1 April 2026, under section 265 of the Finance Act 2026:
| How late | Before | Now |
|---|---|---|
| 1 day | £100 | £200 |
| 3 months | another £100 | another £200 |
| Third late return in a row | £500 | £1,000 (£2,000 after 3 months) |
| 6 months | HMRC estimates the tax and adds 10% of the unpaid tax | |
| 12 months | another 10% of the unpaid tax | |
How does FileMyLtd file the CT600?
- Start from your accounts. The profit comes from the micro-entity accounts you have just approved, so the two filings always agree.
- Adjust for tax. FileMyLtd asks about items treated differently for tax, such as client entertaining, and capital allowances on equipment, with an explanation for each.
- See the tax worked out. Rates, marginal relief, associated companies and short periods are applied, and every step is shown.
- Check, then submit. The return, accounts and computation are validated, then you enter the company UTR and your HMRC login, make the declaration and submit. The login is used for that submission only.
FileMyLtd is in early access, and CT600 filing is being tested with HMRC before it opens. It covers straightforward small companies. Companies with losses carried back, group relief, R&D claims or overseas income should use an accountant.