Many UK residents wonder what is a CT600, why it’s important, and who should file a Company Tax return. Well, completing your Company Tax Return (Form CT600) each year ensures that your company remains compliant with its legal obligations and pays the correct amount of Corporation Tax.
You use this return to determine how much Corporation Tax (CT) you owe and apply any available tax reliefs to reduce your bill. It is often helpful to seek professional advice on how to calculate your taxes accurately, submit your return on time, and avoid interest or penalties.
We keep things simple — fixed fees, no hidden charges, and no surprise bills at the end of the year. Tell us a bit about your limited company and we’ll send you a clear quote straight away.
What is a CT600?
A CT600 is the UK Company Tax Return (Corporation Tax Return), a report used to calculate Corporation Tax for an accounting period and to submit your company’s taxable profits (or losses). UK limited company owners need to optimise their Corporation Tax liability. So, what is a CT600 in simple terms?
It is the primary form that HMRC use to evaluate your company’s Corporation Tax position. It contains essential information regarding your earnings, claimed tax reliefs, and the calculation of your tax liability.
Who Needs to Submit a CT600?
To understand what is a CT600, it is beneficial to know how HMRC uses it to assess Corporation Tax. HMRC requires all trading companies to submit a return for each Corporation Tax accounting period, even if they make a loss or have no tax to pay. “Trading” encompasses:
- Buying or selling
- Renting property
- Advertising
- Employing someone
- Receiving interest
A Corporation Tax accounting period usually lasts 12 months and begins when your company starts trading. It usually aligns with your company’s financial year in the annual accounts. HMRC will send you a notice (CT603) to ensure you submit a return for that period. Organisations that must complete the return include:
- Limited companies registered in the UK
- Foreign companies with a UK branch or office
- Unincorporated members’ clubs, societies and associations
- Trade associations
- Housing associations
- Charities and non-profit organisations
It may not be necessary to submit a complete CT600 if your company is dormant. However, you are required to inform HMRC of your dormancy, and they will determine whether or not you are required to submit a return.
Why is CT600 Important?
Once you know the definition of what is a CT600, it is also important to know its significance. Even if your company did not trade significantly or experience a loss, it is still necessary to submit a return if HMRC has issued a notice. Accurate filing helps avoid penalties, delays, and unnecessary HMRC enquiries. When business owners treat CT600 as “just another form,” they often fail to recognise that taxable profit differs from accounting profit, which is where the problem usually begins.
When Is the Deadline for Submitting the CT600?
As the deadline approaches, understanding what is a CT600 becomes increasingly important, as it is the form that HMRC uses to verify your company’s Corporation Tax position for the year. File your return within 12 months from the end of your accounting period.
The deadline for paying your company tax is typically nine months and one day after the end of the period, and it is distinct from the overall deadline. Therefore, many companies submit their returns before the earlier deadline or often immediately after year-end, so they know how much tax they will be required to pay.
What Information is Required When Filing the CT600?
To complete your CT600 form, you must understand what a CT600 is and what details you need to complete it.
- 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 those you can carry forward from previous years
- Claims for reliefs: using tax breaks for R&D, creative industries, or patented inventions
- CT calculations: deductions, capital allowances, and taxable profit
What Happen If You Submit A CT600 Late?
This will have an effective from on or after 1 April 2026.
| Time after your deadline | Penalty |
| Return late | £200 |
| Return is more than 3 months late | £400 |
| Three successive failures, return late | £1000 |
| Three successive failures, the return is more than 3 months late | £2000 |
How To File A CT600?
After learning what is a CT600, the next step is filing it. You can file your return directly with HMRC or through approved software.
File Through HMRC
Use this service to file your company:
- Company Tax Return (CT600) for Corporation Tax with HM Revenue and Customs (HMRC)
- accounts with Companies House
- Company Tax Return and accounts at the same time if they’re for the same accounting period
The service will convert your accounts to the correct format (iXBRL).
Key Updates: The online filing service will end on March 31, 2026. You can still use it to file and amend your Company Tax Return with HMRC and your Companies House accounts until March 31, 2026. From April 1, 2026, you must use commercial software to prepare annual accounts and company tax returns with HMRC.
File Through Approved Software
Once HMRC closes its online filing tool, approved software will be the only way to submit your CT600 online. Accounting platforms, including Sage or Xero, are already used by numerous businesses to submit their CT600s. These platforms support the process, minimise errors, and ensure adherence to HMRC regulations.
What are the Common Mistakes Businesses Make When Filing a CT600?
A complete understanding of what is a CT600 help you to identify mistakes early on and minimise delays, penalties, and last-minute stress. Here are the mistakes that you must have to avoid:
Confusion Between Taxable Profit and Accounting Profit
After people fully understand what is a CT600, the common error they make is that they assume the profit in the accounts is equivalent to the taxable profit. CT600 is dependent on a tax computation that accounts for disallowable expenses, capital allowances, and reliefs.
Missing Documents
Another significant concern is that people are unaware of supplementary pages (such as CT600A) that apply to specific circumstances. Delays or corrections may result if the appropriate ones are overlooked.
Troubles with the Filing Process
Many companies struggle to understand the CT600 and fail to adhere to the appropriate submission requirements. HMRC’s guidance explains the process of filing returns and supporting documents.
Need Support With CT600 for Your Limited Company?
If you operate a limited company in the UK and want to get expert assistance, we are here for you. Our LimitedCompanyAccountants assist limited companies in completing their CT600 accurately, with correct supporting documents, clear numbers, and no last-minute anxiety. This is how our accountant supports you:
- We accurately file and prepare the CT600
- Provide clear payment guidance and accurate Corporation Tax calculation
- A comprehensive list of the documents that must be submitted
- Help you avoid penalties and late filing issues by carefully tracking deadlines
Whether you’re just forming your company or already knee-deep in paperwork, our London-based accountants are ready to jump in. One quick call and we’ll figure out what you actually need.
The Bottom Line
In the UK, operating a limited company comes with understanding what is a CT600. Most importantly, it should not be treated as a one-page form, but as a complete submission that requires precision. For limited companies, the real risk is not just paying taxes. Instead, the real risk is submitting inaccurate figures, missing documents, or creating avoidable HMRC enquiries.
Getting a professional service from our LimitedCompanyAccountants helps you prepare and file your CT600 correctly, so everything goes smoothly. When your CT600 is handled correctly, you save time, avoid fines, and keep your firm organised for the next year.
Disclaimer: All the information provided in this article is general in nature; it does not intend to disregard any of the professional advice.
