What is a CT600? The UK corporation tax return explained.
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?
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?
Every UK limited company within the charge to corporation tax must file a CT600 — including active trading companies (regardless of profit level), companies that made a loss, dormant companies (unless HMRC has confirmed they don't need to file), property investment companies, and companies with only bank interest income. The only companies that don't file are those where HMRC has specifically confirmed they're dormant and don't need to.
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, charitable donations (Gift Aid), 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
Depending on circumstances, additional pages may be required:
- CT600A — Losses, deficits, and excess amounts
- CT600B — Reliefs and deductions in terms of trades
- CT600C — Group and consortium claims
- CT600D — Insurance and chargeable gains
- CT600E — Charities and CASCs
Most micro-entities only need the core CT600 — no supplementary pages.
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.
- HMRC's own service — HMRC offers a basic online filing service, but it has limitations and doesn't support all company types.
When you file electronically, HMRC returns an IRmark — a digital receipt confirming your submission was received. Keep this.
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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 |
Penalties for late filing
- Initial penalty: £100 (even one day late)
- After 3 months: additional £100
- After 6 months: 10% of unpaid tax
- After 12 months: further 10% of unpaid tax
Common questions
- Do dormant companies need to file a CT600? Yes, unless HMRC has specifically confirmed they don't.
- 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.
Related guides.
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