Last Updated on January 21, 2026 by Joy Kyalo
Once you incorporate a UK company, there are post-registration compliance issues to be aware of, and taxes are one of them. For limited companies, registration for Corporation Tax is required three months after the start of trading, and payments are due annually.
Navigating business taxes can be overwhelming, especially with the various forms to keep track of. I’m sure many business owners have come across the CT600 form. What is CT600? This article will guide entrepreneurs on the CT600, who need it and when it should be submitted to Companies House.
Key Takeaways
- The CT600 is a compulsory annual Corporation Tax Return for all UK limited companies.
- Corporation Tax is charged between 19% and 25% depending on company profits.
- CT600 filing deadlines depend on the accounting period, while tax payment is due earlier.
- Errors or late submissions can lead to penalties, so accuracy and compliance are essential.
What is CT600 Form?
The CT600 form is a Corporate Tax Return that is used by limited companies to submit to Her Majesty’s Revenue & Customs (HMRC). This form is used to notify HMRC of your company’s income, expenses, and tax calculations for the financial year. With this, HMRC can determine how much Corporation tax is owed.
The Corporation Tax rate uses a tiered system depending on the profits the company is making.
- Companies with profits of £50,000 or less – pay 19% tax rate
- Companies with profits of £250,000 or more – pay 25% tax rate
- Companies with profits between £50,000 and £250,000 – claim marginal relief which gradually increases from 19% to 25%.
Information entered into a CT600 includes:
- Company name
- Registration number
- Taxable profits or losses
- Corporation Tax calculations
- Claiming for relief
- A director’s declaration
Who needs to submit CT600?
Businesses registered with Companies House, whether dormant or active, must submit the CT600 form annually. These are limited companies such as;
- Private limited company
- Public limited company
- Limited liability partnership
- Foreign companies that have a permanent UK branch or office.
- Charities and Community Interest Companies (CICs) that have trading income.
Apart from the above, if HMRC requests your company submit a CT600 form, you must ensure it is completed. Do sole traders and partnerships need to file the CT600 form? No, sole traders and partnerships don’t file CT600 forms – they file personal Tax Returns instead.
What information is included in a CT600 form?
Company details – They include the company name, registration number, Unique Taxpayer Reference (UTR), registered office address, and accounting period.
Trading and other income – A limited company must declare all sources of taxable income, which fall broadly into trading profits and other (non-trading) income.
Allowable expenses – They include staff costs, office and property expenses, legal and financial costs, marketing expenses, and equipment expenses.
Capital allowances – These are claimed on your company’s Corporation Tax Return (CT600) to reduce your taxable profits. You must include a separate capital allowances calculation as part of your submission to HMRC.
Losses carried forward and back – To include losses carried forward and back on a Company Tax Return (CT600), you must follow specific procedures for trading losses, property losses, and capital losses, which involve entering details into the relevant boxes of the CT600 form and its accompanying computations.
Corporation tax calculation – This determines the amount of tax a UK limited company owes to HMRC, which is then formally reported using the Company Tax Return (CT600) form.
Director declarations – This declaration confirms that the information provided is correct and complete to the best of the declarant’s knowledge.
How and when to file a CT600
Every company must file a CT600 annually; however, the deadline date varies for each business. The deadline depends on when the company was formed, making it the accounting period.
For instance, if your company is incorporated on 26 December 2025, the accounting period will generally end 12 months after, making the end of the period 25 December 2026. Using this example, the CT600 form will be due by 25 December 2026.
However, it is essential to note that the filing deadline differs from the deadline for paying Corporation Tax. Corporation tax must be paid earlier, that is, within nine months and one day after the end of your accounting period.
So, if your accounting year ends on 25th December 2026, your company’s Corporation tax must be paid by 26 September 2027.
How to file a CT600 form
The CT600 can be filed online or by post. Online filing remains the most efficient process since it is fast and efficient. One can choose to file the CT600 form by themselves, use an accountant, or a company formation agent. Currently, companies with simple tax affairs are allowed by HMRC to file their CT600 online; however, the service is due to close on 31 March 2026. This means that, after March 2026, all companies will need to use commercial software.
The commercial software platform aims to commercial software provide better validation checks, minimising human errors in complex calculations and data entry that can lead to penalties.
To file, one has to;
- Prepare the necessary documents, which include: your profit and loss accounts, balance sheet, reliefs and deductions, any capital allowance, reliefs and deductions.
- Access and download the CT600 form from the HMRC website
- Fill out the form with the accurate company’s financial information
- Review and submit it to the UK government.
It is essential to understand that any errors or minor mistakes can bring about delays and penalties.
Common mistakes to avoid while filing CT600
Missing deadlines: Depending on your accounting period, missing a deadline to file your CT600 return or pay your Corporation Tax results to penalties and interest charges.
Simple errors in company formation: Double-check your company details, such as the company registration number, registered office address, and accounting period, among others.
Claiming ineligible expenses: Errors in expenses can lead HMRC to investigate, which can be costly.
Failing to seek professional advice: The complexities of tax law, which are subject to frequent changes, can make accurate filing difficult for the uninitiated.
Inaccurate income reporting: It is essential to report all sources of income, including interest received or foreign earnings.
Why do businesses outsource CT600 filing?
Improved accuracy: Company formation or accountants ensure the documents or financial records submitted to HMRC are accurate and error-free.
Ensure compliance: With their expertise in Corporate Tax returns, agents and accountants know the deadline of every company to submit the form, depending on their accounting period, ensuring the company does not incur penalties.
Time and resource saving: Keeping track of your business’s financial records and still completing the CT600 form can be time-consuming, bringing about a distraction from the business growth focus.
Cost-effective: Maintaining an in-house team can be costly since you will have to pay them a monthly salary and benefits than outsourcing. When outsourcing, you will only need to pay once annually to get your Corporate tax return filed with HMRC.
Whether you want to do it alone or outsource third-party services to file your CT600, it is essential to know that one has to be very careful not to err or put false information. This may lead to delays or incur penalties.
BusinAssist has partnered with several accountancy firms to ensure the Corporate Tax return is filed accurately and on time to avoid incurring penalties. Contact them for the accountancy requirements at info@businassist.com.
FAQs
Q: Can I file my own CT600?
Ans: Yes, you can file your own CT600 online or on a paper form. If you plan to do it yourself, you must ensure you are accurate to avoid penalties or investigations.
Q: Can you amend a CT600 after 12 months?
Ans: No, you cannot amend a CT600 after the 12-month amendment window has closed. The deadline to amend a Company Tax Return (CT600) is 12 months from the original filing deadline, which typically means you have two years from the end of the accounting period to make an amendment.
Q: Do I need to file CT600 for a dormant company?
Ans: No, not every dormant company needs to file a CT600. Once you have notified HMRC that your company is dormant, you do not need to complete a tax return during the accounting period.
Q: How to submit CT600 to HMRC?
Ans: To file directly through HMRC, go to the File your accounts and Company Tax Return page, and select “Start now”. Once you have logged into your HMRC business tax account and used your company’s authentication code, you’ll be able to fill out a digital form of the CT600 and submit it.
Q: When does CT600 have to be filed?
Ans: Every company must file a CT600 annually, that is, 12 months after the end of the company’s period.
Q: Where to send the CT600 form?
Ans: A completed CT600 form is sent to HMRC to declare the company’s taxable profits for the year.
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Joy is a content writer at BusinAssist with 6+ years of experience in content marketing and social media strategy. She specialises in company formation and virtual operations, guiding entrepreneurs through global markets with clarity, drawing on her marketing and PR background. Business Assist with BusinAssist – your business success partner.