Corporation Tax Deadlines and Penalties for UK Companies Explained (2026)

This guide covers every deadline you need to know, how to work out your specific dates, what happens when you are late, and the full penalty regime that applies from April 2026. If your company's filing deadline falls on or after 1 April 2026, the penalty amounts have doubled for the first time in nearly three decades.

The Three Core Corporation Tax Deadlines Every Company Has

Most directors talk about "the Corporation Tax deadline" as if there is one. There are three. They fall on different dates. They carry different consequences if missed. And filing your accounts with Companies House is not the same as filing your tax return with HMRC, even though the deadlines sit close together.

Here is the full picture for a standard 12-month accounting period:

We'll walk through how to calculate your specific dates below, but first, understand that these three obligations exist separately and carry different consequences if missed.

Weekend and Bank Holiday Adjustments

If your payment deadline falls on a weekend or bank holiday, it is best to pay on the last working day before. HMRC calculates interest from the official due date, not when the payment is made, because payments can take time to process.

How to Calculate Your Corporation Tax Deadlines

Your deadlines run from the end of your Corporation Tax accounting period. In most cases, your accounting period matches your company's financial year. Take the last day of the accounting period, add the relevant period (nine months and one day, or twelve months), and that is your deadline.

First Year Companies: You’ll Usually Need Two CT600 Returns

For Corporation Tax, an accounting period can never be more than 12 months long. HMRC will split any longer period into two separate accounting periods, and you must file a CT600 return for each one.

Why this matters for new companies

When you form a limited company, Companies House sets your first financial year end (the accounting reference date), which often means your first set of statutory accounts covers more than 12 months. HMRC’s rules on Corporation Tax accounting periods don’t follow that same extended period.

How it works in practice

• Your Corporation Tax accounting period starts on the date you first come within the tax charge (usually the incorporation or trading start date).
• If your first financial year runs longer than 12 months, HMRC will automatically split it:
1. First period: 12 months from the start of your tax period.
2. Second period: The remaining days up to your year‑end.

Each of these has its own CT600 filing deadline and Corporation Tax payment deadline. Example: 15-day extension creating two returns

You incorporate on 15 April 2024. Companies House automatically sets your first accounting reference date to 30 April 2025 (matching the month of incorporation but extending to cover just over 12 months). This creates a first financial year of 12 months and 15 days.

HMRC cannot accept a Corporation Tax accounting period longer than 12 months, so it splits this into:

Period 1: 15 April 2024 – 14 April 2025 (exactly 12 months)

• CT600 filing deadline: 14 April 2026
• Payment deadline: 15 January 2026

Period 2: 15 April 2025 – 30 April 2025 (15 days)

• CT600 filing deadline: 30 April 2026
• Payment deadline: 1 February 2026

Important:

• You'll file one set of accounts with Companies House (covering the full 12 months and 15 days) but two separate CT600 returns with HMRC. You must allocate profits and expenses between both periods
• You’ll still receive a Notice to Deliver a Company Tax Return from HMRC confirming your accounting period dates, check it carefully.
• Filing only one return for a period over 12 months can lead to late filing penalties.
• Even if Period 2 shows minimal activity (just 15 days), it still requires a full CT600.

Corporation Tax For Large Companies: Quarterly Instalments

If your company has taxable profits above £1.5 million, the standard 9-months-and-1-day payment deadline does not apply. Instead, you pay Corporation Tax in four quarterly instalments, the first of which falls during the accounting period itself, before the year ends.

For a 12-month accounting period, the instalment dates fall at:

• Month 7 from the start of the accounting period (6 months and 13 days)
• Month 10 (9 months and 13 days)
• Month 13 (12 months and 14 days, i.e. after the year-end)
• Month 16 (15 months and 14 days)

Large companies with profits over £1.5 million must estimate their Corporation Tax mid-year and pay quarterly instalments, with corrections in later payments if needed. HMRC charges interest on any underpaid instalments. The £1.5 million threshold is divided by associated companies. For instance, a company with three associated companies has a £375,000 threshold. Directors should review associated company rules carefully, as they apply even without a formal group structure.

Late Filing Penalties for Corporation Tax: What You Owe & When

Corporation Tax late filing penalties apply automatically. There is no warning letter before the first penalty. The moment your CT600 is one day late, the penalty is issued. Whether you owe any tax is irrelevant: a dormant company with zero tax liability still faces the same fixed penalties for a late return.

Penalties for Filing Deadlines from 1 April 2026

The Government doubled the fixed late filing penalty amounts at Budget 2025. These increases apply to any Company Tax Return with a filing deadline on or after 1 April 2026. The stated reason is that the original penalty amounts, set in the late 1990s, had lost their deterrent effect through inflation.

For returns with a filing deadline before 1 April 2026, the old amounts still apply: £100 at one day, another £100 at three months, then the same percentage-based charges from six months onwards.

Repeat Late Filers

If your company files late for three consecutive accounting periods, the fixed penalty on each of those two initial deadlines (day one and three months) rises to £500 per deadline under the old regime, and to £1,000 per deadline under the new April 2026 regime. A company that misses three returns in a row under the new rules faces fixed penalties of up to £2,000 per return on the two early-stage penalties alone, before any percentage-based charges apply.

Tax Determination at Six Months

If your return is still not filed six months after the deadline, HMRC will write to tell you how much Corporation Tax it thinks you owe. This is called a tax determination. You cannot appeal against a tax determination. You must pay the amount determined and file the return. Once HMRC has the actual return, it recalculates the interest and penalties based on the correct figures. But until then, you owe what HMRC has estimated, which is often higher than your actual liability.

Corporation Tax Late Payment Penalty for: Interest from Day One

Here is something directors sometimes misread: there is no fixed penalty for paying Corporation Tax late. Unlike late filing, HMRC does not issue a £200 or £500 charge simply because the payment arrived after the due date. What HMRC does instead is charge interest, daily, from the day after the payment was due until the day you pay.

From 9 January 2026, the HMRC late payment interest rate is 7.75% per annum. Interest accrues daily, so even a few weeks of delay on a meaningful tax bill adds up.

Penalties for Error in Corporation Tax Return 

Penalties are lower if you disclose the error yourself before HMRC discovers it. HMRC calls this an unprompted disclosure, and it carries a higher reduction than a prompted disclosure (where HMRC has already opened an enquiry). The simplest way to benefit from these reductions is to tell HMRC as soon as you realise something is wrong.

"Careless" means you did not take reasonable care, but you were not intentionally misleading HMRC. It is worth understanding that reasonable care is assessed against your individual circumstances. A company director with accounting experience is held to a higher standard than one who is genuinely unfamiliar with the rules. Getting professional advice before filing a complex return can itself be evidence of reasonable care.

Failure to Notify: Registering for Corporation Tax

A new company must notify HMRC that it is liable for Corporation Tax. HMRC registers most new limited companies for Corporation Tax automatically when they are incorporated, and you receive a letter with your Unique Taxpayer Reference within a few weeks. But if HMRC does not contact you, or if your company starts trading and you have not received confirmation of registration, the obligation to notify sits with you.

The deadline to notify is within three months of starting to trade or becoming active in any way that creates a Corporation Tax liability. If you fail to notify and HMRC later discovers the omission, it can charge a failure to notify penalty. The penalty is calculated as a percentage of the tax that should have been paid, using the same framework as inaccuracy penalties: the worse the behaviour and the longer the delay, the higher the percentage. A reasonable excuse can reduce or eliminate the penalty entirely.

If in doubt about your registration status, the fastest check is to log into your HMRC business tax account. If your company is registered, you will see Corporation Tax as one of your taxes.

Reasonable Excuse: When Penalties Can Be Challenged

You can appeal a late filing penalty if you had a reasonable excuse for missing the deadline. The appeal must be made within 30 days of the penalty notice. You cannot appeal until you have filed the outstanding return. (HMRC guidance: GOV.UK, Company Tax Returns: appeals)

HMRC publishes guidance on what counts as a reasonable excuse. The standard is not low.

What generally qualifies:

• Serious illness or bereavement affecting the person responsible for filing
• HMRC's own errors, such as technical failures on the Government Gateway
• Destruction of records through fire, flood, or another disaster outside your control

What generally does not qualify:

• "My accountant did not get round to it in time"
• "I did not know I had to file"
• "I thought the company was dormant" (dormant companies must still file)
• Cash flow problems preventing payment (this is not a reasonable excuse for late filing, though it may support a Time to Pay request)

The appeal form is completed online and must be submitted in one session, as it cannot be saved and returned to. You will need your company's Unique Taxpayer Reference, the penalty notice date and amount, and a clear explanation of why you could not file on time. Print the completed form and post it to the address shown on the form.

The Joint HMRC and Companies House Filing Service Has Closed

From 1 April 2026, the joint online filing service that allowed companies to file annual accounts and a Company Tax Return together with HMRC and Companies House in a single submission has closed permanently. (Source: HMRC, April 2026)

This means:

• Company Tax Returns must now be filed through commercial software or an accountant
• Annual accounts are still filed separately with Companies House
• Previous returns filed through the old service are no longer accessible through that system

If you used the joint service previously and need copies of old returns, you should have saved copies before the closure. If you did not, you will need to contact HMRC directly to obtain copies.

Conclusion

Corporation Tax comes with three key deadlines: payment is due 9 months and 1 day after the accounting period ends, Companies House accounts are due at 9 months, and the CT600 return is due at 12 months. From 1 April 2026, late filing penalties will double, with a £200 charge for being just one day late. New companies often face two CT600 returns if their first accounting period runs over 12 months.

Although late payment doesn’t incur a fixed penalty, it does accrue interest daily at 7.75% from 9 January 2026. The interest is unavoidable but can be prevented by paying on time. If you're already behind, file as soon as possible it's always better than not filing at all. This will stop penalties from piling up and is necessary to appeal any existing fines.

Frequently Asked Questions