Running a business in the UK means staying on top of many financial responsibilities — and one of the most important is corporation tax. Missing your corporation tax deadlines can lead to penalties, interest charges, and unnecessary stress.
Whether you’re a start-up, small business, or established company, understanding how corporation tax works — and when key deadlines fall — is crucial to keeping your finances in order and avoiding costly mistakes.
In this article, we’ll explain what corporation tax is, when it needs to be paid, how to file your return, and the key deadlines you should never miss.
What Is Corporation Tax?
Corporation tax is a levy on the profits of limited companies and certain other organisations in the UK, such as clubs, co-operatives, and trade associations.
If your business is a limited company registered with Companies House, you are legally required to:
Pay corporation tax on your taxable profits
File a Company Tax Return (CT600) to HM Revenue & Customs (HMRC)
Keep accurate accounting records
Corporation tax applies to:
Trading profits (from business activities)
Investment income
Chargeable gains (from selling assets such as property or shares)
The current corporation tax rate (2025/26) is 25% for companies with profits over £250,000. Companies with profits below £50,000 pay a small profits rate of 19%, and profits in between are subject to marginal relief.
When Do You Need to Pay Corporation Tax?
Unlike many other taxes, corporation tax is not automatically billed by HMRC. It’s your company’s responsibility to:
Calculate how much corporation tax you owe.
Pay it to HMRC by the correct deadline.
File your Company Tax Return.
The timing can be confusing because your payment deadline and filing deadline are different. Let’s break them down.
Corporation Tax Deadlines Explained
The key to staying compliant is knowing two main deadlines:
The payment deadline – when you must pay your corporation tax bill.
The filing deadline – when you must submit your Company Tax Return.
1. Corporation Tax Payment Deadline
You must pay your corporation tax within 9 months and 1 day after the end of your company’s accounting period.
For example:
If your company’s accounting year ends on 31 December 2025, your payment is due by 1 October 2026.
If your year-end is 31 March 2026, your payment is due by 1 January 2027.
It’s important to remember that you need to pay before you file your tax return. HMRC expects the payment first, even if your return is still being prepared.
Large Companies (Quarterly Instalment Payments)
If your taxable profits exceed £1.5 million, you may need to pay corporation tax in quarterly instalments.
These payments are typically due in months 7, 10, 13, and 16 of your accounting period.
For example, if your accounting period runs from January to December, your payments would be due in:
July
October
January
April
Large company instalments help spread the tax burden and keep payments in line with profits.
2. Company Tax Return Filing Deadline
Your Company Tax Return (CT600) must be filed within 12 months of the end of your accounting period.
Example:
Accounting year end: 31 December 2025
Filing deadline: 31 December 2026
You must file your tax return online using HMRC’s online service or compatible accounting software. Paper returns are no longer accepted in most cases.
The return must include:
Your full statutory accounts
Detailed profit and loss figures
Tax calculations and adjustments
Your corporation tax liability
Even if your company has made a loss or owes no tax, you still need to file your return unless HMRC tells you otherwise.
What Happens If You Miss a Corporation Tax Deadline?
Missing your corporation tax payment or filing deadline can result in penalties and interest charges.
Late Filing Penalties
HMRC issues automatic penalties for late submissions:
1 day late – £100 fine
3 months late – another £100 fine
6 months late – HMRC estimates your tax bill and adds a 10% penalty
12 months late – another 10% penalty
If you miss deadlines repeatedly, the £100 fines increase to £500 each.
Late Payment Interest
If you don’t pay your corporation tax on time, HMRC will charge daily interest on the outstanding balance — currently 6.75% (as of 2025).
Interest accrues from the day after the payment deadline until the tax is paid in full.
How to Stay on Top of Corporation Tax Deadlines
Meeting corporation tax deadlines doesn’t have to be stressful. Here are some practical tips to help you stay organised:
Know Your Accounting Period
Your company’s accounting period usually matches your financial year. Double-check it in your HMRC account to confirm your deadlines.Use Accounting Software
Cloud-based systems like Xero, QuickBooks, or Sage can automatically track your deadlines and generate accurate figures for your tax return.Set Calendar Reminders
Mark your key dates (payment and filing deadlines) in your business calendar — ideally with reminders a month in advance.Maintain Accurate Records
Keep all invoices, receipts, and financial statements up to date. Good bookkeeping makes calculating and filing taxes far easier.Work with an Accountant
Partnering with a professional ensures that your corporation tax is calculated accurately and submitted on time. Accountants can also identify tax reliefs and allowances that reduce your tax liability.
Common Corporation Tax Mistakes to Avoid
Even the most diligent business owners can make errors when handling corporation tax. Here are a few pitfalls to watch out for:
Forgetting to register for corporation tax within three months of starting to trade.
Confusing the payment deadline (9 months + 1 day) with the filing deadline (12 months).
Failing to claim allowable expenses or capital allowances, leading to higher tax bills.
Not notifying HMRC when your company becomes dormant.
Leaving it too late to prepare accounts — creating unnecessary pressure and risk of penalties.
Avoiding these mistakes can save time, stress, and money.
How an Accountant Can Help
Corporation tax can be complex, particularly for growing businesses or those with multiple revenue streams. An experienced accountant can:
Calculate your tax accurately and ensure compliance with HMRC rules.
File your Company Tax Return before the deadline.
Identify tax-saving opportunities, reliefs, and deductions.
Manage correspondence with HMRC.
Help plan for future tax liabilities.
This professional oversight not only ensures peace of mind but can also improve cash flow management and long-term profitability.
Conclusion: Stay Ahead of Corporation Tax Deadlines with Expert Support
Staying on top of corporation tax deadlines is an essential part of running a successful business in the UK. Missing key dates can lead to unnecessary penalties and stress — but with proper planning, reliable accounting software, and expert support, you can stay compliant and focused on growth.
At Cox Hinkins, a trusted UK accounting firm, we specialise in helping businesses of all sizes manage their corporation tax obligations with ease. From preparing your Company Tax Return to ensuring your payments are made on time, our experienced accountants handle the details — so you can concentrate on running your business.
Get in touch with Cox Hinkins today to learn how we can help you stay compliant, reduce tax stress, and keep your business financially on track all year round.