Key facts
- The CT600 must be filed within 12 months of the end of the accounting period.
- Corporation Tax must be paid within 9 months and 1 day of the period end.
- Companies House accounts are due within 9 months of the period end (or 21 months for a new company’s first accounts).
- Late filing penalties start at £100 and escalate to 10% of the unpaid tax after 12 months.
CT600 Filing Deadline
The Company Tax Return (CT600) must be filed with HMRC within 12 months of the end of the accounting period it covers. This deadline applies regardless of whether the company has any tax to pay.[1]
| Accounting Period End | CT600 Filing Deadline |
|---|---|
| 31 March 2026 | 31 March 2027 |
| 31 December 2025 | 31 December 2026 |
| 30 June 2026 | 30 June 2027 |
The CT600 must be filed electronically — HMRC does not accept paper Corporation Tax returns. It must be accompanied by the company’s statutory accounts and tax computation, both in iXBRL format.[4] You can submit your CT600 online using HMRC-recognised software such as GoFile.
Payment Deadline
Corporation Tax must be paid within 9 months and 1 day after the end of the accounting period. This is earlier than the filing deadline:[3]
| Accounting Period End | Payment Deadline | CT600 Filing Deadline |
|---|---|---|
| 31 March 2026 | 1 January 2027 | 31 March 2027 |
| 31 December 2025 | 1 October 2026 | 31 December 2026 |
| 30 June 2026 | 1 April 2027 | 30 June 2027 |
In practice, this means you often need to pay your tax bill before you file the return. You should estimate and pay the tax due, then file the formal CT600 afterwards.
Large companies (profits over £1.5 million) must pay Corporation Tax in quarterly instalments during the accounting period itself, not 9 months after it ends. Very large companies (profits over £20 million) must pay even earlier.[3]
Companies House Accounts Deadline
Separately from the CT600, you must also file statutory accounts with Companies House:[4]
| Type of Company | Accounts Filing Deadline |
|---|---|
| Private limited company | 9 months after the accounting reference date |
| Public limited company (PLC) | 6 months after the accounting reference date |
| New company (first accounts) | 21 months from incorporation date |
Late Filing Penalties
HMRC imposes automatic penalties for late CT600 returns. These are fixed-amount penalties that apply regardless of the reason for being late:[2]
| How Late | Penalty |
|---|---|
| 1 day late | £100 |
| 3 months late | Another £100 (total £200) |
| 6 months late | HMRC estimates the tax and adds 10% of the unpaid tax as a penalty |
| 12 months late | Another 10% of the unpaid tax |
If the return is late three consecutive times, the £100 fixed penalties increase to £500 each.[2]
Interest on late payment: In addition to penalties, HMRC charges interest on any Corporation Tax paid after the 9-month-and-1-day deadline. Interest runs from the day after the payment deadline until the date HMRC receives your payment. The rate is set by HMRC and changes periodically.[3]
Complete Deadline Timeline
Using a 31 March 2026 year end as an example:
| Date | Obligation |
|---|---|
| 31 March 2026 | Accounting period ends |
| 31 December 2026 | Companies House accounts filing deadline (9 months) |
| 1 January 2027 | Corporation Tax payment deadline (9 months + 1 day) |
| 31 March 2027 | CT600 filing deadline with HMRC (12 months) |
| 31 March 2027 | Last day to amend the previous year’s CT600 (if filed on time) |
Frequently Asked Questions
When is the deadline for filing a CT600?
The CT600 must be filed with HMRC within 12 months of the end of the accounting period. For a company with a 31 March 2026 year end, the deadline is 31 March 2027.
When must Corporation Tax be paid?
Corporation Tax must be paid within 9 months and 1 day of the end of the accounting period — this is before the CT600 filing deadline. Large companies with profits over £1.5 million must pay in quarterly instalments.
What are the penalties for filing a late CT600?
HMRC charges £100 for being 1 day late, another £100 at 3 months, then 10% of the unpaid tax at 6 months, and a further 10% at 12 months. If your return is late three times in a row, the fixed penalties increase to £500 each.
When are Companies House accounts due?
Private limited companies must file accounts at Companies House within 9 months of their accounting reference date. New companies filing their first accounts get 21 months from incorporation.
Further Reading
- The CT600 Tax Return — what goes into the return
- Amending a CT600 — how to correct mistakes after filing
- Accounting Periods — understanding your period and how long periods are split
- Company Accounts Requirements — what accounts you must prepare and file
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Sources
- Company Tax Returns: deadlines — GOV.UK
- Penalties for late filing of Company Tax Returns — GOV.UK
- Pay your Corporation Tax bill — GOV.UK
- File your accounts and Company Tax Return — GOV.UK