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Corporation Tax Deadline UK: What Limited Companies Need to Know

If you run a limited company in the UK, corporation tax is one of the most important obligations you have to HMRC. Missing the deadline — or misunderstanding when it falls — can result in automatic penalties and interest charges. This article explains exactly when your corporation tax is due, how to calculate what you owe, what happens if you miss the deadline, and how to stay on top of it all without the stress.

What is corporation tax?

Corporation tax is the tax that UK limited companies pay on their taxable profits. It applies to trading profits, investment income, and chargeable gains. Unlike income tax, which is paid by individuals, corporation tax is a company-level obligation — it is entirely separate from the personal tax a director pays on their salary or dividends.

Every limited company registered in the UK must register with HMRC for corporation tax within three months of starting to do business. After that, you must file a Company Tax Return (form CT600) and pay any tax owed for each accounting period — usually every 12 months.

When is the corporation tax deadline?

The corporation tax deadline is not a fixed calendar date like 31 January for self assessment. It depends on your company’s accounting period — specifically, the date your financial year ends.

Here is how the deadlines work:

  • Payment deadline: Nine months and one day after the end of your accounting period.
  • Filing deadline: Twelve months after the end of your accounting period.

So if your company’s accounting period ends on 31 March 2026, your payment is due by 1 January 2027, and your CT600 must be filed by 31 March 2027.

Most small limited companies have a financial year ending on 31 March or 31 December, but this varies. Check your company’s accounting reference date at Companies House if you are unsure.

What counts as an accounting period?

An accounting period for corporation tax purposes cannot exceed 12 months. If your company’s first set of accounts covers more than 12 months — which is common when a company is newly incorporated — HMRC will split it into two separate corporation tax accounting periods. Each period will have its own payment and filing deadlines. This catches many new directors off guard, so it is worth confirming with your accountant early on.

Filing your return vs paying your bill: two separate deadlines

This is one of the most common sources of confusion for limited company directors. You have two distinct deadlines, and they fall at different times.

Payment deadline — nine months and one day

Your corporation tax must be paid to HMRC by nine months and one day after your accounting period ends. HMRC does not send you a bill. You work out what you owe, and you pay it. If you do not have your accounts finalised by that point, you should make a reasonable estimate and pay that amount to avoid interest charges. You can then correct the figure when the return is filed.

Filing deadline — twelve months

Your CT600 (the Company Tax Return) must be filed online with HMRC within 12 months of the end of your accounting period. Your annual accounts are submitted to Companies House on a separate deadline — usually nine months after the year end for private limited companies — but the tax return goes to HMRC.

In practice, you should aim to have your accounts and tax return prepared well before the payment deadline, so you know exactly what you owe before the money is due.

Corporation tax rates in 2026/27

The corporation tax rate you pay depends on the size of your company’s profits. In 2026/27, the rates are as follows:

  • Small profits rate: 19% — applies to companies with taxable profits under £50,000.
  • Main rate: 25% — applies to companies with taxable profits over £250,000.
  • Marginal relief — applies to companies with profits between £50,000 and £250,000. The effective rate tapers between 19% and 25%.

These thresholds are divided by the number of associated companies. So if you own two companies, each company’s thresholds are halved to £25,000 and £125,000 respectively. This is an area where many directors are caught out, so always check the position with your accountant if you have more than one company.

What counts as taxable profit?

Taxable profit is not the same as your accounting profit. You start with the profit shown in your accounts and then make adjustments — adding back any disallowable expenses and deducting any reliefs such as capital allowances or R&D tax credits. Good bookkeeping throughout the year makes this process far more straightforward and reduces the risk of errors.

How to calculate your corporation tax liability

There is no single formula that works for every company, but the general process looks like this:

  1. Start with your net profit from your profit and loss account.
  2. Add back any expenses that are not allowable for tax purposes (such as client entertaining or depreciation).
  3. Deduct capital allowances on qualifying assets (in place of depreciation).
  4. Deduct any other reliefs, such as R&D tax credits or losses carried forward from previous years.
  5. Apply the relevant corporation tax rate to arrive at your tax liability.

This is where a good set of management accounts during the year is genuinely useful. If you track your figures quarterly, you can estimate your year-end tax position in advance and put money aside rather than scrambling for it after the year ends.

Allowable expenses — a quick guide

HMRC allows you to deduct expenses that are incurred wholly and exclusively for business purposes. Common allowable expenses include:

  • Staff wages and employer’s National Insurance contributions
  • Office rent and business rates
  • Business insurance premiums
  • Professional fees, including accountancy fees
  • Business travel (not ordinary commuting)
  • Subscriptions to professional bodies
  • Advertising and marketing costs

Expenses that are not allowable include depreciation, client entertaining, and any costs with a dual personal-business purpose where the personal element cannot be separated.

Large companies and quarterly instalment payments

If your company’s annual taxable profits exceed £1.5 million, the standard nine months and one day payment deadline does not apply. Instead, you must make quarterly instalment payments (QIPs) throughout the year.

The four instalments fall in months 7, 10, 13, and 16 of the accounting period, meaning you start paying before the year has even ended. This applies to large companies and very large companies (with profits over £20 million), with the latter required to pay even earlier.

Most small and medium-sized limited companies will never reach these thresholds, but it is worth knowing the rule exists if your company is growing rapidly.

Penalties and interest for missing the deadline

HMRC takes late payment and late filing seriously. Here is what you face if you miss the deadlines.

Late filing penalties

If you miss the 12-month filing deadline for your CT600, HMRC applies automatic penalties:

  • 1 day late: £100 flat penalty.
  • 3 months late: A further £100 penalty.
  • 6 months late: HMRC will estimate your tax liability and charge 10% of the unpaid tax as a penalty.
  • 12 months late: A further 10% of the unpaid tax is charged.

These penalties increase further if a company has a history of late filing.

Late payment interest

Interest accrues on any unpaid corporation tax from the day after the payment deadline. HMRC’s interest rate on late payments is set above the Bank of England base rate — check the latest HMRC guidance for the current rate, as it changes with base rate movements. Interest is not a penalty that can be appealed; it is a statutory charge that simply accrues until the debt is paid.

Deliberate errors and fraud

If HMRC believes your return contains deliberate errors or that you have withheld information, penalties can reach 100% of the unpaid tax. This is rare for well-run small companies, but it underlines why accurate records and honest reporting matter.

Practical tips to stay on track

Deadlines rarely sneak up on companies that plan ahead. Here are the habits that make the biggest difference.

Know your accounting period end date

Write down your accounting period end date and count forward nine months and one day (payment) and 12 months (filing). Set calendar reminders for both. If you are unsure of your accounting reference date, check your Companies House filing history.

Set aside tax throughout the year

A rough rule of thumb: set aside around 19–25% of your monthly profits into a separate savings account throughout the year. This means the money is available when the payment deadline arrives rather than having to find a large lump sum at short notice. Reviewing your cash flow regularly will help you stay on top of this.

Get your accounts prepared early

The earlier your accountant has your records, the earlier your accounts and tax return can be completed. Aim to hand over your records within four to six weeks of your year end. This gives you plenty of time to review the figures, understand your liability, and pay before the nine-month deadline.

Use accounting software

Cloud-based accounting software such as Xero keeps your records up to date in real time. If you are not already using it, Xero training can help you and your team get the most from the software and reduce the time your accountant spends on data reconciliation. Good software also makes it easier to produce in-year reports that give you an early view of your likely tax position.

Do not wait until year end to think about tax planning

Decisions that affect your corporation tax bill — such as timing of capital expenditure, director salary levels, and pension contributions — should be made before the year end, not after. Once the year has closed, your options are limited. A quarterly review with your accountant gives you the opportunity to adjust your plans while there is still time to act.

Pay on account if your accounts are not ready

If your accounts will not be finalised before the payment deadline, estimate your liability conservatively and pay that amount to HMRC. This stops interest accruing on the estimated amount. Once your return is filed, HMRC will either request the balance or refund any overpayment. It is always better to pay too much and receive a refund than to underpay and face interest.

Whether you are a new director filing your first corporation tax return or an established business owner looking to tighten up your processes, the principle is the same: plan ahead, keep your records clean, and engage your accountant early. The deadlines are fixed and HMRC will not waive interest simply because you forgot. Build the tax calendar into your financial planning and it becomes a routine task rather than an annual crisis.

Frequently asked questions

When is corporation tax due for a limited company?

Corporation tax must be paid nine months and one day after the end of your company’s accounting period. For example, if your year ends on 31 December 2025, payment is due by 1 October 2026. The filing deadline for the CT600 tax return is 12 months after the accounting period ends — in this example, 31 December 2026.

What happens if I miss the corporation tax payment deadline?

HMRC charges interest on any unpaid corporation tax from the day after the payment deadline. The interest accrues daily until the debt is cleared. There is no grace period — interest begins immediately. Check the latest HMRC guidance for the current interest rate, as it moves with the Bank of England base rate.

What is the penalty for filing a company tax return late?

HMRC charges a £100 automatic penalty if your CT600 is filed even one day late, with a further £100 if it is three months late. At six months late, a penalty of 10% of the unpaid tax is added, and a further 10% is charged at 12 months late. Filing late also damages your compliance record with HMRC, which can lead to higher penalties in future years.

Do I still need to file a corporation tax return if my company made no profit?

Yes. Even if your company made a loss or broke even, you are still required to file a CT600 with HMRC. You should also consider filing to preserve any losses, which can be carried forward and set against future profits to reduce your tax liability in later years.

Can I pay corporation tax in instalments?

Most small limited companies must pay their full corporation tax bill in one payment by the nine-month-and-one-day deadline. Quarterly instalment payments only apply to large companies with annual taxable profits above £1.5 million. If you are struggling to pay, contact HMRC before the deadline to discuss a Time to Pay arrangement — HMRC is generally more willing to help if you get in touch proactively.

What is the corporation tax rate in 2026/27?

In 2026/27, the small profits rate is 19% on profits under £50,000 and the main rate is 25% on profits over £250,000. Companies with profits between £50,000 and £250,000 pay an effective rate between 19% and 25% through marginal relief. These thresholds are divided between associated companies, so owning multiple companies affects the rates that apply to each one.