Your company tax return (CT600) must be filed with HMRC within 12 months of your accounting period end, but Corporation Tax itself must be paid earlier, 9 months and 1 day after that date. Missing either deadline triggers penalties or interest. Large companies with profits above £1.5 million pay Corporation Tax in quarterly instalments rather than one lump sum.
Your company tax return (CT600) must be filed with HMRC within 12 months of the end of the accounting period it covers, even if your company made a loss or owes no Corporation Tax.
Corporation Tax payment is due earlier than the filing deadline: 9 months and 1 day after the accounting period end for companies with annual profits under £1.5 million.
Companies with taxable profits above £1.5 million must pay Corporation Tax in quarterly instalments, while very large companies with profits above £20 million pay instalments in the current accounting year.
Late filing penalties start at £100 on day one, add a further £100 at three months, then 10% of unpaid tax at six months, and a further 10% at twelve months, regardless of whether any tax is owed.
First-year accounts may cover up to 18 months, but a Corporation Tax accounting period cannot exceed 12 months, so two CT600 returns must be filed for any period longer than that.
The deadline for filing your company tax return is 12 months after the end of the accounting period it covers.
Frequently asked questions
When is my company tax return due?
Your CT600 company tax return must be filed with HMRC within 12 months of the end of your accounting period. For a company whose accounting period ends on 31 March 2026, the filing deadline is 31 March 2027. HMRC will issue a notice to file (form CT603), but you are obliged to submit the return on time regardless of whether you receive that notice. Failure to file on time triggers automatic financial penalties.
Is the Corporation Tax payment deadline the same as the filing deadline?
No, and this is a common source of missed payments. Corporation Tax must be paid 9 months and 1 day after your accounting period end, three months before the filing deadline. A company with a 31 March 2026 year end must pay by 1 January 2027 but has until 31 March 2027 to file. Interest accrues from the day after the payment due date, so the bill grows even while the return is still within its filing window.
What happens if I file my company tax return late?
HMRC charges a £100 penalty the day after the filing deadline is missed. A further £100 is charged if the return is still outstanding after three months. At six months, a tax-geared penalty of 10% of any unpaid Corporation Tax is added, with another 10% at twelve months. Companies that file late three times in a row see the initial flat penalties rise to £500. Interest also accrues separately on any Corporation Tax not paid by the payment deadline.
Do I have to file a company tax return if I owe no Corporation Tax?
Yes. Every active UK limited company must file a CT600 for each accounting period, even when the company made a loss, claimed reliefs reducing the liability to zero, or is in its first year of trading. HMRC issues a notice to deliver (CT603), but the obligation to file exists whether or not that notice arrives. Dormant companies may also need to file if HMRC has issued a notice. Missing the deadline triggers the full penalty ladder regardless.
When does a large company have to pay Corporation Tax?
Companies with annual taxable profits above £1.5 million must pay Corporation Tax in quarterly instalment payments rather than one lump sum. The four payments fall in months 7, 10, 13 and 16 of the accounting period. Very large companies with profits above £20 million pay on an accelerated schedule, with instalments falling in months 3, 6, 9 and 12 of the current accounting year. Getting the instalment amounts wrong can attract interest charges even before the year end.
What is the deadline for a company's first tax return?
A company's first accounts can cover up to 18 months from incorporation, but a Corporation Tax accounting period cannot exceed 12 months. If your first accounts span more than 12 months, HMRC splits the period and requires two CT600 returns: one covering the first 12 months, a second for the remainder. Each carries its own filing deadline and payment deadline, so two Corporation Tax payments may fall due before the first filing deadline arrives.
What are the Corporation Tax rates for 2025/26?
For the 2025/26 tax year the small profits rate is 19% on annual profits up to £50,000, and the main rate is 25% on profits above £250,000. Companies with profits between £50,000 and £250,000 pay a blended effective rate under marginal relief, which tapers between the two rates. These thresholds are divided proportionally if the accounting period is shorter than 12 months or if the company has associated companies. The rates are unchanged from April 2023.
How do I file my company tax return with HMRC?
All CT600 returns must be filed electronically through HMRC's Corporation Tax online service or approved commercial software. The statutory accounts and tax computations submitted alongside the CT600 must be tagged in iXBRL format, which allows HMRC to read the figures automatically. Paper filing is only available in exceptional circumstances. Most limited company directors use an accountant or dedicated filing software to handle iXBRL tagging, since errors in the tagged accounts can delay processing or trigger HMRC queries.
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