Filing a Corporation Tax return is one of the most important annual responsibilities for any UK limited company. Whether your business turned a profit, broke even, or made a loss, HMRC still expects a return. Understanding the 2026 rules, deadlines and rates will help you stay compliant, avoid penalties and pay only what you genuinely owe.

This guide walks you through what a Corporation Tax return is, who needs to file one, the current 2025/26 and 2026/27 rates, and the reliefs that could lower your tax bill.

What Is a Corporation Tax Return?

A Corporation Tax return, formally known as the CT600, is the document UK companies submit to HMRC each year. It declares your taxable profits, the reliefs you are claiming, and the Corporation Tax you owe for a specific accounting period.

The return must be filed online using HMRC approved software or the HMRC online service. It is normally prepared after your statutory accounts have been finalised, because the figures in both documents must reconcile.

Who Must File a Corporation Tax Return in the UK?

You must submit a CT600 if your business is a UK incorporated company. This includes:

• Private limited companies (Ltd)
• Public limited companies (PLC)
• Foreign companies with a UK branch or office
• Members clubs, societies, associations and other unincorporated bodies that fall within the charge to Corporation Tax

Even dormant companies that have not traded may need to file a return if HMRC has issued a notice to deliver one. If you are unsure, contact HMRC or speak to your accountant.

Sole traders and ordinary partnerships do not file a CT600. They report profits through Self Assessment instead.

Corporation Tax Deadlines You Need to Know

There are two separate deadlines, and many directors confuse them. Missing either one can trigger penalties or interest charges.

Payment deadline

Corporation Tax is payable 9 months and 1 day after the end of your accounting period. For a company with a year end of 31 March 2026, payment is due by 1 January 2027.

Large companies with profits over £1.5 million must usually pay in quarterly instalments during the accounting period.

Filing deadline

Your CT600 return must be filed with HMRC within 12 months of the end of your accounting period. So for the same 31 March 2026 year end, the CT600 is due by 31 March 2027.

In practice, most accountants prepare the return well before the payment deadline, since you cannot accurately pay the right amount of tax without first calculating it.

UK Corporation Tax Rates for 2025/26 and 2026/27

The rates that applied from April 2023 continue to apply for the financial year beginning 1 April 2026. There are no announced rate changes, so the structure remains as follows:

How Marginal Relief works

If your profits fall between £50,000 and £250,000, you pay the 25% main rate but receive Marginal Relief, which gradually reduces your bill. The standard fraction used in the calculation is 3/200.

The practical effect is that each additional pound of profit inside the relief band is taxed at an effective marginal rate of 26.5%, which is actually higher than the 25% main rate above £250,000. This is worth knowing before you draw extra dividends or accept a one off contract that pushes you into the band.

Associated companies reduce your thresholds

If your company has associated companies, broadly meaning other companies under common control, the £50,000 and £250,000 limits are divided between them. Two associated companies share the thresholds equally, so each gets £25,000 and £125,000. Dormant companies are usually excluded from the count.

This is one of the most common Corporation Tax errors directors make, especially when they own multiple property SPVs or family companies.

How to File Your CT600 Return

Corporation Tax returns must be filed electronically. You have two main options:

1. HMRC online service — suitable for straightforward small companies
2. HMRC approved commercial software — used by accountants and most growing businesses for accuracy and integration with bookkeeping systems

You will also need to file your statutory accounts with Companies House and, in most cases, send a copy of those accounts and your tax computations to HMRC alongside the CT600.

Information You Need to Prepare a CT600

To prepare an accurate Corporation Tax return, gather the following:

• Company statutory accounts for the period
• Profit and loss figures and balance sheet
• Capital allowances claimed on equipment, vehicles or qualifying assets
• Adjustments for disallowable expenses such as client entertaining
• Details of any reliefs being claimed (R&D, Patent Box, loss relief)
• Records of group relief or losses brought forward
• Details of dividends paid and director loan balances
• Bank statements, sales invoices and purchase records

Your tax return must reconcile with your filed statutory accounts. Any differences should be clearly explained in the tax computation.

Common Reliefs That Reduce Corporation Tax

Many companies pay more Corporation Tax than they need to simply because they overlook available reliefs. The most widely used include:

• Capital allowances on qualifying plant, machinery, vans and equipment, including the Annual Investment Allowance and Full Expensing
• Research and Development (R&D) tax relief for companies developing new products, services or processes
• Patent Box relief, which applies a 10% effective rate to profits attributable to qualifying patents
• Loss relief, allowing trading losses to be carried back, carried forward or surrendered to group companies
• Creative industry tax reliefs for film, video games, theatre, animation and similar sectors

The rules for each relief are detailed and HMRC scrutinises R&D claims more closely than ever, so accurate record keeping and correct categorisation are essential.

Penalties for Late or Incorrect Returns

HMRC issues automatic penalties for missing the filing deadline:

• 1 day late: £100
• 3 months late: a further £100
• 6 months late: HMRC estimates your tax bill and adds a 10% penalty
• 12 months late: another 10% penalty on the unpaid tax

If you are late three times in a row, the £100 penalties rise to £500 each.

Late payment of Corporation Tax also attracts interest, charged daily from the day after the payment deadline. Errors and inaccuracies can lead to further penalties, with the size depending on whether HMRC considers the error careless, deliberate or concealed.

Record Keeping Requirements

HMRC expects companies to keep their records for at least 6 years from the end of the accounting period they relate to. Records you should retain include:

• Books of account and ledgers
• Bank statements and finance agreements
• Sales and purchase invoices
• Payroll and PAYE records
• VAT records, if registered
• Evidence supporting reliefs and capital allowances claimed

If HMRC opens a compliance check, these records will form the backbone of your defence.

When Professional Help Is Worth It

Corporation Tax has become more complex since the rate changes of 2023, the introduction of Full Expensing, and tighter scrutiny of R&D claims. Companies with associated entities, international activities, property portfolios or ambitious growth plans benefit most from professional tax advice.

A good accountant does more than file the CT600. They forecast where your profits will fall against the £50,000 and £250,000 thresholds, time capital purchases to maximise allowances, and advise on dividend versus salary strategies before year end, not after.

Key Takeaways

• Every UK limited company must file a CT600 each year, even dormant ones if HMRC requests it
• Corporation Tax is due 9 months and 1 day after your accounting period ends
• The CT600 must be filed within 12 months of period end
• Rates for 2025/26 and 2026/27: 19% below £50,000, 25% above £250,000, with Marginal Relief in between
• The effective rate inside the Marginal Relief band is 26.5%
• Associated companies share the profit thresholds, which can change your effective rate
• Reliefs such as capital allowances and R&D tax credits can significantly reduce your bill

Need Help With Your Corporation Tax Return?

Whether you are filing your first CT600, dealing with associated companies, or want to make sure you are claiming every relief available, speaking to a qualified accountant pays for itself many times over.

Target Accounting UK supports limited companies, contractors and property businesses across the UK with end to end Corporation Tax compliance, planning and HMRC representation. Get in touch for a no obligation conversation about your company’s tax position.