Corporation Tax Deadlines UK: When to File and Pay in 2025

If you're running a UK limited company, understanding your corporation tax deadline UK is critical to staying compliant with HMRC. Missing these deadlines can result in penalties, interest charges, and unwanted stress. This comprehensive guide covers everything you need to know about corporation tax deadlines, payment schedules, current rates, and how to avoid costly mistakes.

What Is the Corporation Tax Payment Deadline?

The most important date to remember is this: you must pay your corporation tax 9 months and 1 day after your accounting period ends.

For example, if your company's accounting period ends on 31 March 2025, your corporation tax payment is due by 1 January 2026.

This deadline applies to most UK limited companies. However, large companies with profits over £1.5 million have different rules and must pay quarterly instalments (more on this later).

Key Corporation Tax Dates at a Glance

Corporation Tax Filing Deadline UK

Whilst you need to pay your corporation tax 9 months and 1 day after your accounting period ends, you have slightly longer to file your CT600 return.

The filing deadline is 12 months after your accounting period ends.

Using the same example above, if your accounting period ends on 31 March 2025:

• Payment due: 1 January 2026 (9 months and 1 day)
• CT600 filing due: 31 March 2026 (12 months)

This means you must pay your corporation tax before you file your return. You'll need to estimate your tax liability or work with your accountant to calculate the correct amount.

What Is a CT600 Return?

The CT600 is HMRC's Corporation Tax Return form. It includes:

• Your company's profits and losses
• Corporation tax calculation
• Details of any reliefs or allowances claimed
• Supporting financial accounts and tax computations

Most companies file their CT600 online through HMRC's Corporation Tax Online service or via commercial accounting software.

Corporation Tax Rates 2025

As of 2025, the UK operates a tiered corporation tax system based on your company's profits:

Main Rate: 25%

Companies with profits over £250,000 pay the main rate of 25% on all profits.

Small Profits Rate: 19%

Companies with profits of £50,000 or less pay a reduced rate of 19%.

Marginal Relief: £50,000 to £250,000

If your company's profits fall between £50,000 and £250,000, you'll pay corporation tax somewhere between 19% and 25%. This is calculated using marginal relief, which gradually tapers the rate as profits increase. Use our corporation tax calculator to see your exact liability.

The effective tax rate in this range depends on your exact profit level. For instance:

• £75,000 profit: approximately 21% effective rate
• £150,000 profit: approximately 23% effective rate
• £200,000 profit: approximately 24% effective rate

Important: These thresholds are divided by the number of associated companies plus one. If you have associated companies, your thresholds will be lower.

Understanding Your Accounting Period

Your accounting period is usually the same as your financial year, typically 12 months. However, it can be shorter (but never longer than 12 months for corporation tax purposes).

When Does Your Accounting Period Start?

For most companies, your first accounting period begins on the date of incorporation. Subsequent periods usually run for 12 months from the end of the previous period.

You can choose your year-end date, but many companies align it with:

• 31 March (aligns with UK tax year)
• 31 December (calendar year)
• Anniversary of incorporation

Timeline Example: Year-End 31 December 2024

Let's walk through a complete timeline for a company with an accounting period ending 31 December 2024:

1 January 2024 - 31 December 2024 Accounting period (trading year)

31 March 2025 HMRC sends a notice to file your CT600 return (within 3 months of period end)

1 October 2025 PAYMENT DEADLINE - Corporation tax must be paid (9 months and 1 day after 31 Dec 2024)

31 December 2025 FILING DEADLINE - CT600 return and accounts must be submitted (12 months after period end)

Timeline Example: Year-End 31 March 2025

For companies with a financial year ending 31 March 2025:

1 April 2024 - 31 March 2025 Accounting period

30 June 2025 HMRC sends notice to file

1 January 2026 PAYMENT DEADLINE - Pay your corporation tax

31 March 2026 FILING DEADLINE - Submit CT600 return

Quarterly Instalment Payments for Large Companies

If your company has profits over £1.5 million, you're classified as a "large company" and must pay corporation tax in quarterly instalments.

Large Company Payment Schedule

Large companies pay their corporation tax in four instalments:

1. Month 7 of the accounting period (estimated payment)
2. Month 10 of the accounting period (estimated payment)
3. Month 1 after the period ends (estimated payment)
4. Month 4 after the period ends (final payment)

For example, for an accounting period from 1 April 2024 to 31 March 2025:

• 14 October 2024 (month 7)
• 14 January 2025 (month 10)
• 14 April 2025 (month 1 after period ends)
• 14 July 2025 (month 4 after period ends)

Note: The £1.5 million threshold is divided by the number of associated companies plus one. Two associated companies would mean each has a threshold of £500,000.

Very Large Companies

Companies with profits exceeding £20 million must pay instalments earlier, starting in month 3 and month 6 of the accounting period, then months 9 and 12.

Penalties for Late Corporation Tax Payment

HMRC takes late payment seriously. Here's what happens if you miss your deadline:

Interest Charges

Interest starts accruing immediately from the day after the payment deadline. The interest rate is set by HMRC and typically runs at 2.5-4% above the Bank of England base rate.

As of 2025, the late payment interest rate is approximately 7.75% per year. This compounds daily, so delays get expensive quickly.

Example Interest Calculation

If you owe £10,000 in corporation tax and pay it 3 months late:

• Daily interest rate: 7.75% ÷ 365 = 0.021%
• Daily charge: £10,000 × 0.021% = £2.10
• 90 days late: £2.10 × 90 = £189 in interest charges

No Fixed Penalties for Late Payment

Unlike late filing (covered below), there are no fixed penalties for late payment of corporation tax itself. However, the interest charges can mount up significantly.

Penalties for Late Filing of CT600

Filing your CT600 return late triggers automatic penalties, even if you've already paid your corporation tax.

Late Filing Penalty Structure

Example Penalty Calculation

Company owes £20,000 corporation tax and files return 13 months late:

• 1 day late: £100
• 3 months late: £100
• 6 months late: £2,000 (10% of £20,000)
• 12 months late: £2,000 (another 10%)
• Total penalties: £4,200 (plus interest on the tax itself)

Repeated Late Filing

If you file late for three consecutive years, the fixed penalties double to £200 for the third year and increase further for subsequent years.

How AccountsOS Tracks Your Corporation Tax Deadline

Managing corporation tax deadlines manually can be stressful, especially when you're running a business. AccountsOS automatically tracks your critical deadlines so you never miss a payment or filing date.

Automatic Deadline Tracking

When you set up your company in AccountsOS, the system:

• Records your accounting period end date
• Calculates your corporation tax payment deadline (9 months and 1 day)
• Calculates your CT600 filing deadline (12 months)
• Sends timely reminders before each deadline

Smart Notifications

AccountsOS sends you notifications at strategic intervals:

• 3 months before payment deadline: Start preparing and calculating your liability
• 1 month before payment deadline: Final reminder to pay
• 2 weeks before payment deadline: Urgent reminder
• Filing deadline reminders: Similar notification schedule for CT600 submission

Real-Time Tax Estimates

Throughout your accounting period, AccountsOS provides real-time estimates of your corporation tax liability based on your current profits. This helps you:

• Budget for your tax payment
• Avoid surprises when the deadline approaches
• Make quarterly provisions if needed

Integration with Your Accountant

AccountsOS makes it easy to share information with your accountant:

• Export financial data with one click
• Share access to your AccountsOS workspace
• Ensure both you and your accountant have the same deadline information

How to Calculate Your Corporation Tax

Calculating corporation tax involves several steps:

Step 1: Calculate Taxable Profits

Start with your accounting profit (revenue minus expenses), then make adjustments:

• Add back disallowable expenses (entertainment, depreciation, fines)
• Deduct capital allowances (see our guide to allowable business expenses)
• Apply any trading losses from previous years

Step 2: Apply the Correct Tax Rate

• Profits ≤ £50,000: Apply 19% rate
• Profits > £250,000: Apply 25% rate
• Profits between £50,000 and £250,000: Calculate using marginal relief formula

Step 3: Apply Reliefs and Credits

Deduct any applicable reliefs:

• R&D tax credits
• Patent box relief
• Creative industry tax reliefs
• Double taxation relief

Example Calculation: £100,000 Profit

A company with £100,000 taxable profit (in the marginal relief band):

1. Calculate standard corporation tax at 25%: £100,000 × 25% = £25,000
2. Calculate marginal relief: Complex formula based on profit level
3. Marginal relief reduces tax by approximately: £2,400
4. Corporation tax due: £25,000 - £2,400 = £22,600 (effective rate: 22.6%)

Frequently Asked Questions

When is corporation tax due in the UK?

Corporation tax must be paid 9 months and 1 day after your accounting period ends. For example, if your year-end is 31 March 2025, payment is due by 1 January 2026.

Can I pay corporation tax in instalments?

Smaller companies (profits under £1.5 million) must pay the full amount in one lump sum. Large companies with profits over £1.5 million must pay quarterly instalments throughout and after the accounting period.

What happens if I file my CT600 late?

Late filing triggers automatic penalties starting at £100 if you're even one day late. Penalties increase significantly after 3, 6, and 12 months, potentially adding thousands of pounds to your tax bill.

How do I pay corporation tax?

You can pay corporation tax through:

• Direct debit (set up through your HMRC online account)
• Bank transfer (using your unique payment reference)
• CHAPS same-day payment
• Debit or corporate credit card online

Always ensure you use your correct corporation tax payment reference (17 digits starting with your company number).

When do I need to register for corporation tax?

You must register for corporation tax within 3 months of starting to trade. HMRC usually registers you automatically when you incorporate your company through Companies House, but you should verify this in your HMRC online account.

What is the corporation tax rate for 2025?

For the 2025 tax year, the rates are:

• 19% for profits up to £50,000 (small profits rate)
• 25% for profits over £250,000 (main rate)
• Tapered rate (19-25%) for profits between £50,000 and £250,000 (marginal relief applies)

Can I claim a refund if I overpay corporation tax?

Yes. If you overpay corporation tax, HMRC will either refund you automatically or offset the amount against future tax liabilities. Refunds typically process within a few weeks of filing your CT600 return, but you can claim interest if HMRC delays unreasonably.

Do I need to pay corporation tax if my company made a loss?

No. If your company made a loss, you don't pay corporation tax for that period. However, you still must file a CT600 return to report the loss. You can carry the loss forward to reduce future profits or, in some cases, carry it back to previous years to claim a refund. Our year-end tax planning guide covers loss utilisation strategies in detail.

Stay Ahead of Your Corporation Tax Deadlines

Missing corporation tax deadlines is expensive and stressful. Between payment deadlines, filing deadlines, and penalty structures, there's a lot to track.

AccountsOS eliminates the stress by automatically monitoring your deadlines, estimating your tax liability in real-time, and sending timely reminders. Our AI-powered platform keeps you compliant whilst you focus on growing your business.

Whether you're managing one company or multiple entities, AccountsOS ensures you never miss a critical HMRC deadline again. From automatic receipt capture to intelligent tax calculations, we've built the accounting system small businesses deserve. See how AccountsOS compares to traditional accountants or explore our features.

Ready to stop worrying about deadlines? Start your free AccountsOS trial today and experience automated accounting that works as hard as you do.