How Corporation Tax deadlines work
UK Corporation Tax does not follow the standard tax year (6 April to 5 April) that applies to individuals. Instead, your Corporation Tax obligations are tied to your company's accounting period, which is usually 12 months starting from your date of incorporation or your chosen accounting reference date.
This means every company has different deadlines. Your deadlines depend entirely on when your accounting period ends.
Key deadlines for every accounting period
For each accounting period, there are two critical deadlines:
| Obligation | Deadline | What happens if you miss it |
|---|---|---|
| Corporation Tax payment | 9 months and 1 day after accounting period end | Interest charged from the due date |
| CT600 tax return filing | 12 months after accounting period end | Automatic £100 penalty, escalating with delay |
These are separate deadlines. You must pay the tax you owe before you file the return. Many directors confuse the two and assume filing the return is the only obligation.
Example: Accounting period ending 31 March 2026
If your company's accounting period ends on 31 March 2026:
- Corporation Tax payment due: 1 January 2027
- CT600 return due: 31 March 2027
If your accounting period ends on 31 December 2025:
- Corporation Tax payment due: 1 October 2026
- CT600 return due: 31 December 2026
Corporation Tax rates for 2026
The current Corporation Tax rates apply based on your taxable profits:
| Profit level | Rate |
|---|---|
| Up to £50,000 (small profits) | 19% |
| £50,001 to £250,000 (marginal relief) | Effective rate between 19% and 25% |
| Over £250,000 | 25% |
If your company is part of a group or has associated companies, the thresholds are divided by the number of associated companies. A company with one associated company would have its small profits threshold halved to £25,000.
Filing the CT600
The CT600 is your Corporation Tax return. It must be filed online with HMRC and includes:
- Your company's taxable profits or losses
- Corporation Tax due
- Capital allowances claimed
- Any reliefs or deductions applied
- Details of loans to participators (if applicable)
- Full statutory accounts (iXBRL tagged)
- Tax computations
You file the CT600 through HMRC's online services or through compatible accounting software. The return must include your accounts in iXBRL format, which most accounting packages generate automatically.
Penalties for late filing
Late filing of the CT600 triggers automatic penalties:
| Delay | Penalty |
|---|---|
| 1 day late | £100 |
| 3 months late | Additional £100 |
| 6 months late | HMRC estimates your tax and adds 10% of that amount |
| 12 months late | Additional 10% of estimated tax |
These penalties apply even if you have already paid your Corporation Tax on time. Filing and payment are separate obligations with separate penalty regimes.
If your return is late three times in a row, the £100 penalties increase to £500 each.
Late payment interest
If you pay your Corporation Tax late, HMRC charges interest from the day after the payment deadline. The interest rate is set at the Bank of England base rate plus 2.5%. As of early 2026, this means late payment interest is significant and accumulates daily.
There is no penalty for late payment of Corporation Tax for most companies (only interest). However, large companies with profits over £1.5 million must pay in quarterly instalments, and different rules apply.
Quarterly instalment payments
Companies with taxable profits exceeding £1.5 million in an accounting period must pay Corporation Tax in four quarterly instalments rather than a single payment. The instalments are due in months 7, 10, 13, and 16 of the accounting period.
For very large companies with profits over £20 million, instalments begin earlier: months 3, 6, 9, and 12.
How Incorra helps
Incorra calculates your Corporation Tax deadlines automatically based on your accounting period. The tax dashboard shows your estimated liability, upcoming payment dates, and filing deadlines. You receive alerts well before each deadline, giving you time to prepare your accounts and arrange payment.
[Start using Incorra](https://incorra.com/auth/register) to track your Corporation Tax deadlines automatically.