Filing your CT600 Corporation Tax return is a crucial annual obligation for every limited company in the UK. In the 2026/26 tax year, over 2.5 million companies are expected to file their returns with HMRC, making it essential to understand the process and deadlines to avoid penalties.
| Key Corporation Tax Figures 2026/27 | Amount |
|---|---|
| Corporation Tax main rate | 25% |
| Small profits rate (profits up to £50,000) | 19% |
| CT600 filing deadline | 12 months after end of accounting period |
| Corporation Tax payment deadline | 9 months and 1 day after accounting period ends |
Understanding the CT600 Corporation Tax Return
The CT600 is the official form used by limited companies to report their taxable profits and calculate the corporation tax due. All UK limited companies must file a CT600 return for each accounting period, even if no tax is due or the company has made a loss.
Filing this return accurately ensures compliance with HMRC rules and enables proper calculation of the tax liability. The CT600 form is submitted online via HMRC's Corporation Tax service, which requires your company’s Unique Taxpayer Reference (UTR) and a Government Gateway account.
When to File Your CT600
The CT600 return must be filed within 12 months of the end of your company’s accounting period. For most companies, the accounting period aligns with the financial year recorded at Companies House.
Corporation tax itself must be paid sooner — within 9 months and 1 day after the end of the accounting period. Missing the filing deadline can result in penalties starting at £100, increasing with time and the size of your company’s profits.
For example, if your accounting period ended on 31 March 2026, you must submit your CT600 by 31 March 2026 and pay your corporation tax by 1 January 2026.
Preparing Your Accounts and Financial Information
Before completing your CT600, you need to prepare your company’s statutory accounts. These accounts include your profit and loss statement, balance sheet, and notes to the accounts. They form the basis for calculating your taxable profit.
Your accounts must comply with UK accounting standards and be filed with Companies House. The figures from these accounts are then adjusted for tax purposes — for example, certain expenses may be disallowed or capital allowances applied.
Ensure your accounting period for corporation tax matches your financial year to simplify reporting. If it doesn't, you will need to complete multiple CT600 returns covering different periods.
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Step-by-Step Guide to Filing Your CT600
Filing your CT600 involves completing the form accurately and submitting it online via HMRC’s Corporation Tax online service or compatible accounting software.
Step 1: Register for Corporation Tax
When you set up your limited company, you must register for Corporation Tax within 3 months of starting to trade or becoming active. You can do this on GOV.UK using your company’s Unique Taxpayer Reference (UTR), which HMRC sends after incorporation.
Step 2: Gather financial information
Collect your statutory accounts and ensure all adjustments for tax purposes are identified. This includes:
- Taxable profits or losses
- Capital allowances
- Disallowed expenses
- Any reliefs or deductions
Step 3: Complete the CT600 form
You can complete your CT600 directly through HMRC’s online service, or via compatible accounting software that submits returns digitally. The form includes sections for:
- Company details
- Accounting period dates
- Tax calculation and payable amount
- Claims for reliefs or losses
Step 4: Review and submit
Double-check all entries for accuracy. Mistakes or omissions can trigger enquiries or penalties. Once satisfied, submit your CT600 electronically. HMRC will send a confirmation of receipt.
Step 5: Pay your Corporation Tax
Pay the tax due within 9 months and 1 day after the accounting period ends. You can pay online through HMRC’s website via bank transfer, debit card, or other accepted methods.
Common Mistakes to Avoid When Filing
Even experienced company directors can slip up when filing the CT600. Here are some frequent errors to watch out for:
- Missing the filing deadline: Late submissions result in automatic penalties starting from £100.
- Incorrect accounting period dates: Ensure the accounting period matches your company’s statutory accounts.
- Failing to include all income: Declare all taxable income and gains to avoid investigations.
- Not claiming reliefs: Losses or capital allowances not claimed can increase your tax bill unnecessarily.
- Using outdated forms or software: Always use the latest CT600 form for the relevant tax year.
For more detailed guidance on Corporation Tax, see our Corporation Tax Guide.
What to Do After Filing Your CT600
After submitting your CT600, keep a copy of the submitted return and confirmation email for your records. HMRC may contact you for clarifications or to request further information.
Ensure you pay the corporation tax by the deadline to avoid interest or penalties. If you anticipate difficulties with payment, contact HMRC promptly to discuss instalment options.
Remember to file your company accounts with Companies House as well — this is a separate legal requirement. For help understanding the differences between sole traders and limited companies, check out Sole Trader vs Limited Company.
- The CT600 must be filed within 12 months of your accounting period end; corporation tax is due earlier (9 months + 1 day).
- Prepare accurate statutory accounts and adjust for tax before completing your CT600.
- Submit your return online using HMRC’s service or compatible software.
- Avoid common errors like missing deadlines, incorrect dates, and failing to claim reliefs.
- Keep clear records and pay your corporation tax promptly to stay compliant.
Can I file my CT600 return on paper?
No, HMRC requires all CT600 returns to be filed online for accounting periods starting on or after 1 April 2019. Paper returns are no longer accepted for most companies.
What happens if I file my CT600 late?
You will incur automatic penalties starting at £100 for late filing. Additional fines can apply depending on how late the return is and your company’s taxable profits. Interest may also be charged on unpaid tax.
How do I find my company’s Unique Taxpayer Reference (UTR)?
Your company’s UTR is a 10-digit number sent by HMRC shortly after incorporation. You can find it on official HMRC correspondence or by logging into your HMRC online account.
Official Sources
* GOV.UK: Set up a business · * HMRC: Income Tax rates · * HMRC: Corporation Tax · * HMRC: VAT registration