Corporation Tax Deadlines Calendar 2026

Missing a Corporation Tax deadline costs real money — late filing penalties start at £100 and escalate fast, while late payment interest accrues daily. This complete 2026 calendar lays out every key date so you can plan ahead and avoid unnecessary charges.

How Corporation Tax deadlines work

Before the calendar, let's recap how deadlines are calculated. Every company's deadlines are based on its accounting period end date (not the calendar year or tax year):

• Corporation Tax payment: Due 9 months and 1 day after the end of the accounting period
• CT600 filing: Due 12 months after the end of the accounting period
• Penalties kick in: The day after the filing deadline

If a deadline falls on a weekend or bank holiday, the deadline moves to the next working day.

For a detailed breakdown of filing deadlines, see our CT600 deadline guide.

2026 month-by-month calendar

The tables below show deadlines falling in each month of 2026, based on the most common accounting period end dates.

January 2026

New Year's Day note: 1 January 2026 is a bank holiday (Thursday). Deadlines falling on this date shift to Friday 2 January 2026 for payment purposes. HMRC's online services may have reduced availability.

February 2026

February is relatively quiet for Corporation Tax. Use this month to prepare if your company has a 31 March year-end — your payment will be due in January and your CT600 by 31 March.

March 2026

31 March is one of the busiest dates in the Corporation Tax calendar. It's the most popular year-end for UK companies, so HMRC's systems can experience heavy traffic. Don't leave filing to the last day.

April 2026

April rate changes: Check whether Corporation Tax rates or thresholds change from 1 April 2026. If your accounting period straddles 1 April, you may need to apportion profits between two financial years at different rates. See our Corporation Tax rates guide for current and upcoming rates.

May 2026

June 2026

Mid-year review: If your company has a 31 March year-end, June is a good time to do a mid-year tax review. Assess your projected profits and plan any year-end tax planning strategies.

July 2026

August 2026

Summer holiday warning: Many directors take August holidays and forget about impending deadlines. If your year-end was 31 October 2025, your tax payment is due 1 August — set a reminder before you go away.

September 2026

October 2026

Companies with 31 December year-ends: Your tax payment is due 1 October, but your CT600 filing isn't due until 31 December. Don't wait until filing to calculate what you owe — use a Corporation Tax calculator to estimate your liability and pay on time.

November 2026

December 2026

Christmas period: HMRC's online systems remain available over Christmas, but telephone support has reduced hours. If you need to contact HMRC about your Corporation Tax account, do so before mid-December.

Quarterly instalment dates for large companies

If your company's Corporation Tax liability exceeds £150,000 (or the threshold is reduced for associated companies), you must pay in quarterly instalments during the accounting period rather than waiting until 9 months after.

For companies with a 31 March 2026 year-end, the quarterly instalment dates are:

For companies with a 31 December 2025 year-end:

Very large companies (profits over £20 million) must pay even earlier — instalments start in the third month of the accounting period. See our quarterly instalments guide for the full rules.

Late filing penalties

If you miss your CT600 filing deadline, HMRC's penalty structure is:

If you file late for three consecutive periods, the £100 penalties increase to £500 each.

You can appeal a late filing penalty if you have a reasonable excuse, but HMRC sets a high bar. "I forgot" or "I didn't know" won't usually succeed.

For the full breakdown, see our penalties guide.

Late payment consequences

Late Corporation Tax payments attract:

• Interest — charged from the day after the payment deadline at HMRC's current rate (check GOV.UK for the latest rate, which changes periodically)
• Surcharges — for very late payments or repeated late payment, HMRC may apply additional surcharges

Interest is calculated daily and compounds. Even being a week late on a £10,000 liability costs you several pounds in interest — and for larger amounts, the costs add up quickly.

See our guide on Corporation Tax interest for current rates and how to calculate what you owe.

Payment methods and processing times

When paying close to a deadline, choose your payment method carefully:

Tip: Always use your correct payment reference number when paying. The wrong reference can mean your payment isn't allocated to your account, and you'll appear to have paid late.

For full instructions, see our guide on how to pay Corporation Tax.

Key dates to remember for 31 March year-end companies

Since 31 March is the most common year-end, here's a focused summary for these companies:

For the year ending 31 March 2026:

For the year ending 31 March 2025 (filing in 2026):

How to avoid missing deadlines

1. Set calendar reminders

Add your payment and filing deadlines to your calendar with reminders at:

• 3 months before (start preparing)
• 1 month before (finalise accounts)
• 1 week before (file and pay)

2. Use accounting software

Modern accounting software tracks your deadlines automatically. Taxpipe shows your upcoming deadlines on your dashboard and sends email reminders as they approach.

3. File early

There's no advantage to waiting until the deadline. File your CT600 as soon as your accounts are ready. Many companies file within a few months of their year-end, giving them a comfortable buffer.

4. Pay early if in doubt

If your accounts aren't finalised but you have a rough idea of your Corporation Tax liability, consider paying an estimated amount before the deadline. You can claim a refund if you overpay, and HMRC will pay you interest on the overpayment.

5. Appoint a tax agent

An accountant or tax agent can manage deadlines for you, receiving HMRC reminders and handling filings on your behalf.

Special situations

New companies

Your first accounting period might not align with a standard 12-month cycle. Check your specific dates carefully — see our new company timeline guide for worked examples.

Companies changing their year-end

If you change your accounting period, your deadlines change too. The transitional period will have its own unique payment and filing dates.

Companies being struck off or dissolved

If your company is being closed down, you still need to file a final CT600 covering the period up to the date of cessation. Don't assume dissolution removes the filing obligation.

Frequently asked questions

What if the deadline falls on a weekend?

If a Corporation Tax payment or filing deadline falls on a Saturday, Sunday, or bank holiday, the effective deadline is the next working day. However, it's always safer to file or pay the working day before.

Can I get an extension to file my CT600?

HMRC doesn't grant extensions for CT600 filing. The 12-month deadline is considered generous compared to other taxes. If you genuinely cannot file on time, file as soon as possible to minimise penalties and consider appealing on the basis of a reasonable excuse.

Do I need to file if my company made a loss?

Yes. You must file a CT600 regardless of whether your company made a profit or loss. A loss return is important because it establishes your loss carry-forward position for future years.

When does HMRC's interest start on late payments?

Interest accrues from the day after the payment deadline. If your payment was due on 1 January 2026 and you pay on 15 January 2026, you owe 14 days of interest.

Is there a deadline for amending a CT600?

Yes — you can amend a CT600 up to 12 months after the original filing deadline. For a 31 March 2025 year-end, the filing deadline is 31 March 2026, so the amendment deadline is 31 March 2027. See our CT600 amendments guide for the process.

What happens if my company is dormant?

If HMRC has been notified that your company is dormant, you won't need to file CT600s until you start trading. However, you must notify HMRC promptly when the company becomes active again.

Related guides

• Corporation Tax Deadlines 2026: When You Need to File and Pay
• CT600 Deadline: When Is Your Company Tax Return Due?
• CT600 Penalties: What Happens If You File Late?
• How to Pay Corporation Tax to HMRC
• CT600 Deadline: When to File Your Corporation Tax Return

Related Reading

• Corporation Tax Deadlines 2026: When You Need to File and Pay
• The Cheapest Way to File Your CT600 in the UK (2026)
• Corporation Tax Payment: Deadlines, Methods & What Happens If You're Late