When Is My Corporation Tax Deadline? Key Dates Every UK Company Must Know

Missing a corporation tax deadline is one of the most expensive mistakes a UK company can make. Between late filing penalties, interest charges, and HMRC enquiries, getting your dates wrong could cost your business thousands of pounds — and all of it is entirely avoidable.

Whether you have just incorporated your first company or you have been running a business for years, this guide breaks down every key corporation tax deadline you need to know for the 2024/25 tax year and beyond.

Understanding Your Accounting Period

Before diving into deadlines, you need to understand your accounting period. This is the 12-month period your company uses to calculate its profits and losses. For most companies, it aligns with one of these:

• 1 April to 31 March (matching the tax year)
• 1 January to 31 December (calendar year)
• The anniversary of your incorporation date

Your accounting period determines every other deadline. HMRC does not use a single fixed date for all companies — your deadlines are unique to your company.

The Two Key Deadlines

There are two critical corporation tax deadlines, and they are different:

1. Corporation Tax Payment Deadline

You must pay your corporation tax bill within 9 months and 1 day after the end of your accounting period.

Example: If your accounting period ends on 31 March 2025, your payment deadline is 1 January 2026.

This is the deadline most companies trip up on, because HMRC charges interest from day one if you pay late. There is no grace period. The current interest rate on late payments is 7.25% (as of 2024), which adds up quickly.

2. CT600 Filing Deadline

You must file your Company Tax Return (CT600) within 12 months after the end of your accounting period.

Example: If your accounting period ends on 31 March 2025, your CT600 filing deadline is 31 March 2026.

Note that the filing deadline is 3 months after the payment deadline. This means you are expected to pay your tax bill before you have even filed your return. Many directors find this counterintuitive, but it is how the system works.

Quarterly Instalment Payments for Large Companies

If your company's taxable profits exceed £1.5 million (or a proportionally lower threshold if you have associated companies), you must make quarterly instalment payments (QIPs) rather than paying in one lump sum.

The four instalment dates are:

• 6 months and 13 days after the start of the accounting period
• 9 months and 13 days after the start
• 14 days after the end of the accounting period
• 3 months and 14 days after the end

For a company with a 1 April 2024 to 31 March 2025 accounting period:

Instalment	Due Date
1st payment	14 October 2024
2nd payment	14 January 2025
3rd payment	14 April 2025
4th payment	14 July 2025

Very large companies (profits over £20 million) must pay even earlier, with the first instalment due in the third month of the accounting period.

Penalties for Late Filing

HMRC takes late filing seriously. The penalty structure for CT600 returns is:

• 1 day late: £100 penalty
• 3 months late: Another £100 penalty (£200 total)
• 6 months late: HMRC estimates your tax bill and adds a penalty of 10% of the unpaid tax
• 12 months late: Another 10% of unpaid tax penalty

If you file late three times in a row, the initial £100 penalties increase to £500 each.

These penalties are in addition to any interest charged on late payment.

Penalties for Late Payment

Late payment does not trigger fixed penalties in the same way, but you will be charged interest on the outstanding amount from the day after the deadline. HMRC may also charge a surcharge if payment is significantly overdue.

The interest rate is set at base rate plus 2.5%, which currently makes it one of the most expensive forms of borrowing available.

How to Avoid Missing Your Deadlines

Here are practical steps to ensure you never miss a corporation tax deadline:

Set calendar reminders well in advance. Do not wait until the deadline month. Set reminders at:

• 6 months before your payment deadline
• 3 months before your payment deadline
• 1 month before your payment deadline
• The same intervals for your filing deadline

Prepare your accounts early. The single biggest reason companies miss deadlines is that their accounts are not ready in time. Start gathering your records at least 3 months before your year-end.

Use accounting software. Modern cloud accounting tools can automate much of the preparation work and send you deadline reminders.

File and pay electronically. HMRC requires CT600 returns to be filed online. Electronic payment methods (BACS, CHAPS, or online banking) are faster and more reliable than cheques.

Consider a professional. An accountant or tax adviser can manage your compliance calendar and ensure nothing falls through the cracks.

What If You Have Already Missed a Deadline?

If you have already missed a deadline, act immediately:

1. File your return as soon as possible — penalties increase the longer you wait
2. Pay any outstanding tax — interest accrues daily
3. Contact HMRC if you have a reasonable excuse — they may cancel penalties for genuine circumstances such as serious illness, bereavement, or natural disasters
4. Appeal if necessary — you have 30 days from the penalty notice to appeal

How TaxDocs Can Help

Keeping track of corporation tax deadlines, preparing the supporting documentation, and ensuring accuracy across your tax computations is time-consuming — especially if you operate across multiple jurisdictions. TaxDocs automates the generation of corporation tax documents, helping you stay organised and compliant. Upload your financial data, and TaxDocs produces professionally formatted, jurisdiction-specific tax documents in minutes rather than hours.

Quick Reference: Your Deadlines at a Glance

Task	Deadline
Pay corporation tax	9 months + 1 day after accounting period end
File CT600 return	12 months after accounting period end
First accounts (new company)	21 months after incorporation
Notify HMRC you are active	Within 3 months of starting to trade

Stay on top of your deadlines, and you will avoid unnecessary costs that eat into your profits.

This article is for informational purposes only and does not constitute tax advice. Always consult a qualified accountant for advice specific to your circumstances.