What is CT600? Understand your Company Tax Return

Introduction

Completing your Company Tax Return (form CT600) each year ensures you stay on top of your legal obligations and pay the right tax. On this return, you’ll calculate how much Corporation Tax (CT) you owe, using any relevant reliefs to reduce your bill. This article explains the CT600 form and who needs to file it. Find expert tips on how to work out your tax accurately and submit your return efficiently and on time to avoid interest or penalties.

What is a CT600 form?

As a UK limited company owner, it’s crucial to optimise your Corporation Tax liability. The Company Tax Return (form CT600) is where you make an accurate calculation. It’s your company’s formal statement of taxable profits for the financial year, and includes crucial information about your earnings, claimed tax reliefs, and the workings behind your tax liability.

Who needs to file a CT600?

HMRC requires every trading company to file a return for each CT accounting period, even if you make a loss or have no company tax to pay. “Trading” includes:

• Buying or selling
• Renting property
• Advertising
• Employing someone
• Receiving interest

Your CT accounting period is normally 12 months and starts when your company begins trading. It usually correlates with the financial year in your annual accounts. HMRC will send you a notice (CT603) to deliver a return for that period. You must comply with this notice.

Overseas companies with a UK branch or office, and some clubs, societies, associations, charities, and community sports clubs also need to file a CT600.

Unincorporated sole traders and partnerships do not submit a CT600. They report income through their Self Assessment return.

If your company is dormant – that is, not trading or receiving income – you may not need to send a full CT600. But you need to notify HMRC of dormancy, after which they will confirm whether or not you need to submit a return. 1st Formations’ dormant company accounts service can offer support.

When is the CT600 filing deadline?

You must file your return within 12 months from the end of your accounting period. There’s a separate, earlier deadline for paying your company tax – it’s usually nine months and one day after the end of the period. So, in practice, most companies fill in their return before the earlier deadline, and often immediately after their year-end, so they know how much they’ll owe.

For example, if your accounting year ends on 31 October 2025, your CT payment is due by 1 August 2026, while your CT600 filing deadline is 31 October 2026. You or your accountant would likely complete the return and make your calculations well before August.

What information goes into a CT600?

To complete your CT600, you’ll need details of your:

• Company information – such as the registered name, company number, and Unique Taxpayer Reference
• Financial results – showing income, and taxable profits or losses for the period, or that you can carry forward from previous years
• Claims for reliefs – for example, using tax breaks for R&D, creative industries, or patented inventions
• CT calculations – including workings

How to work out what you owe

You don’t get a bill for CT – you must compute your own liability in CT600. HMRC provides guidance on working out your CT. But if you’re unsure about any part of the process, speak to a professional accountant or tax adviser.

Helen Wood, Technical Content Writer at TaxAssist, says to calculate what you owe, start with your company’s profit from its accounts, then apply CT allowances and reliefs where relevant. “The contents of the return depend on what your business has done and how tax breaks apply,” she says. For instance, if you spend on capital items in the year, you can potentially claim capital allowances spread over several years.

What is the difference between CT600 and CT603?

CT600 is the tax return, and CT603 is the notice legally requiring you to file that return for a specified accounting period, together with your accounts and computations. HMRC usually issues a notice after you register a limited company or when your accounting period ends.

How to file a CT600 online

You can currently file your return directly with HMRC or through approved software. This will change on 1 April 2026, after which you can only use software.

File directly

HMRC provides an online service for companies with straightforward tax affairs. Visit File your accounts and Company Tax Return and click ‘Start now’. Once you’ve signed into your business tax account and entered your company’s authentication code, you can complete CT600 and submit it online.

Use approved software

When HMRC retires its filing tool, software will be the only way to file online. Many businesses already use accounting platforms, such as Sage or Xero, to submit their CT600, as these platforms help streamline the process, reduce errors, and ensure compliance with HMRC requirements.

In full accounting suites, you can record income and expenses in real time, manage invoices, handle payroll, and generate financial reports. This means that, when it’s time to file your CT600, much of the necessary information is already present and organised.

If you don’t use an accounting platform already, Wood advises exploring your options as soon as possible, so you can pick a solution and get used to using it before April 2026.

The advantages of using software extend far beyond more efficient Company Tax Returns, she says. “It should make business admin a quicker and more integrated process rather than a big, frustrating task at month-end and year-end involving papers and receipts flying everywhere. For example, an invoicing app on your phone can enable you to get paid, immediately improving cash flow.”

Late filing penalties and appeals

If you don’t submit your CT600 form on time, HMRC will issue penalties. If it’s late by:

• One day, the fine is £100
• Three months, another £100 fine is added
• Six months, HMRC may estimate your CT and apply a 10% penalty on unpaid tax
• 12 months, it can charge an additional 10%

If you have a reasonable excuse for missing the deadline, such as illness or bereavement, you may be able to appeal a penalty.

Susan Perry, Accountant and Founder at Cooper Accountancy, says: “Not complying with HMRC rules can result in legal action, debt collection, or the Revenue determining your tax, which could be more than what your company may owe. Your company may be stopped from carrying forward any losses, and it could damage your ability to get finance or contracts, or even sell the company.’’

Common mistakes to avoid when filing

Perry says common errors to watch out for in the CT600 process are:

• Counting expenses that are not tax deductible, such as entertaining and depreciation. “Make sure you understand the disallowable expenses rules,” she says. “Just because your company has paid for something, doesn’t necessarily mean it can lower your tax liability.’’
• Not including taxable income
• Not claiming correctly for capital allowances
• Mistaking the filing deadline for the payment deadline
• Not carrying losses back or forward
• Using the wrong CT rate, or not applying marginal relief for profits between £50,000 and £250,000. Use HMRC’s calculator to check you’re using the correct rate.

Your Company Tax Return: A step-by-step guide

1. Prepare records – gather your company accounts, profit and loss, and details of reliefs and allowances.
2. Confirm accounting period – check the start and end dates of your Corporation Tax period.
3. Work out taxable profits – start with the profit in your accounts, then apply allowances and reliefs.
4. Complete CT600 – enter company details, financial results, tax breaks, and Corporation Tax calculations.
5. File online – submit via HMRC’s online service (until 31 March 2026) or approved software.
6. Pay Corporation Tax – settle your bill within nine months and one day after your period ends.
7. Keep records – retain your CT600, accounts, and supporting documents for at least six years after the end of the accounting period.

Mastering your company tax return

You can file a CT600 on your own, but it’s best to do this only if you understand all parts of the form, and you’re certain all your figures are correct. HMRC provides guidance to help you through the process.

To stay compliant, make sure you maintain accurate financial records and use calendar reminders to track important dates and deadlines. Seek professional help for anything you’re uncertain about. An adviser, such as an accountant, can give you peace of mind that you’re fully compliant, and help you make savings or improve your long-term business finances.

Want to ensure your compliance is handled properly? 1st Formations’ Full Company Secretary Service can help you stay on top of your obligations.