What is a CT600? The UK corporation tax return explained.
Short version. If you're a director of a UK limited company, you'll need to file a CT600 — but what exactly is it, and what does it involve?
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CT600 in one sentence
The CT600 is the form (technically, the electronic return) that UK limited companies use to report their income, expenses, and corporation tax liability to HMRC. Think of it as the company equivalent of a personal Self Assessment tax return.
Who needs to file a CT600?
A company must deal with an HMRC notice to deliver a Company Tax Return, even if it made a loss or owes no tax. If it is dormant, tell HMRC and arrange withdrawal of any notice that is no longer needed. Dormancy does not automatically cancel an outstanding return. See HMRC's dormant company guidance.
What does a CT600 cover?
- Company information — company name and UTR, accounting period dates, type of company and nature of business (SIC code).
- Income and profits — trading profits/losses, investment income, chargeable gains, property income, other income.
- Deductions and reliefs — trading losses brought forward or carried back, capital allowances, qualifying charitable donations, marginal relief, other reliefs.
- Tax calculation — taxable profits after deductions, corporation tax rate applied (19%, 25%, or marginal), marginal relief calculation, total corporation tax due, tax already paid, net amount due/repaid.
Supplementary pages
We support CT600A for prepared loans and qualifying Part 2 repayments within nine months, including amendments. See how to add CT600A, preview and file.
Depending on circumstances, additional pages may be required:
- CT600A: close company loans and arrangements to confer benefits on participators
- CT600B: controlled foreign companies, foreign permanent establishment exemptions, hybrid and other mismatches
- CT600C: group and consortium relief
- CT600D: insurance
- CT600E: charities and Community Amateur Sports Clubs
Supplementary pages depend on the transactions, not simply company size. Check the HMRC forms collection. This list describes HMRC forms, not a promise that every page is supported by this service.
What gets filed alongside the CT600?
- Statutory accounts (iXBRL format) — balance sheet, profit and loss account, any notes, submitted in iXBRL. HMRC doesn't accept PDFs.
- Tax computation — shows how you arrived at taxable profit and corporation tax liability, bridging accounting profit to taxable profit.
How is a CT600 filed?
CT600 returns must be filed electronically — HMRC doesn't accept paper returns. The return is submitted in XML format through HMRC's Government Gateway. Options:
- Tax filing software — tools like SimpleCompanyTax generate the CT600 XML and iXBRL accounts and submit them electronically.
- Through an accountant — most accountants use professional software to file on your behalf.
- The former joint online service is closed. The HMRC and Companies House accounts and Company Tax Return service closed on 31 March 2026. Use suitable software for the Company Tax Return. Separate Companies House services are a different matter. See the official closure notice.
When you file electronically, HMRC returns an IRmark — a digital receipt confirming your submission was received. Keep this.
File your CT600 online
HMRC-compliant XML + iXBRL accounts in one flow. From £10 +VAT per company, per year — £10 +VAT dormant, £25 +VAT micro-entity.
CT600 deadlines
| What | Deadline |
|---|---|
| CT600 filing | 12 months after accounting period end |
| Corporation tax payment | 9 months and 1 day after accounting period end |
| Companies House accounts | 9 months after accounting period end |
A CT600 can only cover up to 12 months. If your company's first accounting period is longer than that, you normally file two CT600s for it. See your first year: filing over 12 months. If the company was dormant and started trading later, in a first year or a later one, HMRC may expect a single CT600 from the trading start instead, but only where its records show that date: see dormant at first, then trading.
Penalties for late filing
For return filing dates on or after 1 April 2026:
- Initial penalty: £200 (even one day late)
- After 3 months: additional £200
- After 6 months: 10% of unpaid tax
- After 12 months: further 10% of unpaid tax
Three consecutive late returns increase each flat penalty to £1,000. Earlier filing dates use the earlier schedule. See the dated penalty guide and HMRC's penalties guidance.
Common questions
- Do dormant companies need to file a CT600? Deal with any outstanding notice to deliver. Once HMRC knows the company is dormant, another return is not normally needed unless HMRC requests one.
- What if my company made a loss? You still file — losses can be carried forward.
- Can I file CT600 myself? Yes, with the right software. Can I file company tax myself?
- What if I miss the deadline? Penalties apply immediately.