- What Is a Company Tax Return (CT600)?
- Who Needs to File a Company Tax Return?
- CT600 Filing and Corporation Tax Payment Deadlines
- What documents do I need to file my company tax return using CT600?
- How to File Your Company Tax Return: Step by Step
- Common Mistakes to Avoid When Filing Your UK Company Tax Return
- Late Filing Penalties and Interest in 2026
- What Changed for CT600 Filing in 2026
- Filing Extensions and Special Circumstances
- FAQs: Frequently Asked Questions
- Conclusion
To file a UK company tax return, prepare your statutory accounts and Corporation Tax computation, complete the CT600 in HMRC recognised software, and submit it online with iXBRL tagged attachments within 12 months of your accounting period end. Pay any Corporation Tax earlier, 9 months and 1 day after period end. Late filing now costs from £200.
This guide covers who must file, the documents you need, the steps, the 2026 penalties and the mistakes that trigger HMRC enquiries. Whether you’re submitting for the first time or trying to improve your existing approach, this blog will help you navigate the CT600 with confidence and prevent costly mistakes.
What Is a Company Tax Return (CT600)?
A Company Tax Return (CT600) is the formal document that UK companies must submit to HMRC to report their profits and calculate the corporation tax they owe. It gives specific information on a company’s financial performance, such as revenue, allowed expenses, and taxable earnings. All limited companies, regardless of size, are required to file a CT600 on an annual basis, even if they owe no taxes. Failure to file or submit incorrect information can result in penalties. A full return has three parts: the CT600 form, your statutory accounts and your Corporation Tax computation, with the accounts and computation in iXBRL format.
Who Needs to File a Company Tax Return?
Any UK company or organisation subject to Corporation Tax that receives a formal “notice to deliver a Company Tax Return” from HMRC must file a Company Tax Return (form CT600), even if it makes no profit or owes no tax. This normally includes most trading limited companies and other bodies, but excludes sole traders and ordinary partnerships, which submit Self Assessment returns.
Core rule: notice from HMRC
- A company or association must file a Company Tax Return if HMRC issues a “notice to deliver a Company Tax Return” to its registered office.
- The obligation still applies whether the company incurred a loss or has no Corporation Tax to pay. A nil return is still needed.
Types of entities that usually file
- UK limited companies (Ltd): Most private limited firms registered with firms House that conduct business must register for Corporation Tax and file a Company Tax Return each year.
- Foreign companies with a UK presence: Overseas companies with a UK branch or office that are subject to UK Corporation Tax may be required to submit a CT600.
- Certain non‑profit bodies and clubs: Companies and some associations with taxable income (such as trading profits or investment income) must file if HMRC notifies them.
Do I need to file a Company Tax Return if my company is dormant?
A dormant company is usually not required to file a company tax return once HMRC has confirmed it is dormant for Corporation Tax, but there are key circumstances and exceptions.
When a dormant company does not file:
If HMRC has been formally informed that the company is dormant for Corporation Tax purposes and acknowledges this, the company is not required to pay Corporation Tax or file further Company Tax Returns for the entirely dormant periods.
For this to apply, the company must be completely non-trading and have no taxable revenue (such as interest, rent, or trade receipts) throughout the accounting period.
When a dormant company must still file
- If HMRC issues a “notice to deliver a Company Tax Return,” the company is required to file a return even if it is dormant or does not owe Corporation Tax.
- If the company traded during the accounting period before becoming dormant, it must file a company Tax Return for the time it was active and pay Corporation Tax on any taxable profits.
Other filings that still apply
- Even when a limited company is dormant for Corporation Tax purposes, it is still required to file annual dormant accounts and a confirmation statement with Companies House.
- These streamlined “dormant company accounts” must be filed on time each year to avoid late-filing penalties and potential strike-off.
If your company becomes active again:
- When a dormant company resumes trading or receives taxable income, HMRC must be notified promptly.
- From that time on, the company must submit Company Tax Returns and pay any Corporation Tax payable for the active periods.
CT600 Filing and Corporation Tax Payment Deadlines
In 2026, UK companies must pay Corporation Tax 9 months and 1 day after year-end, file the CT600 within 12 months of year-end, and file accounts with Companies House within 9 months of the accounting reference date (private companies).
| Example: 31 March 2026 year end | Deadline |
|---|---|
| Companies House accounts (private company) | 31 December 2026 |
| Corporation Tax payment | 1 January 2027 |
| CT600 filing | 31 March 2027 |
- CT600 Filing Deadline: Your Company Tax Return (CT600) must be filed within 12 months of the end of your company’s accounting period. For example, if your accounting period ends on 31 March 2026, your CT600 must be filed by 31 March 2027.
- Corporation Tax Payment Deadline: Corporation Tax owed must be paid before the CT600 filing date. The deadline is 9 months and 1 day after the end of your accounting period. In the same example, tax for the 31 March 2026 year end is due on 1 January 2027. Companies with taxable profits above £1.5 million pay in quarterly instalments, which fall earlier.
Important Filing Requirements:
- CT600 returns must be submitted electronically using HMRC recognised commercial software. HMRC’s free online filing service closed on 31 March 2026.
- Paper returns are accepted only in limited cases, such as filing in Welsh or where HMRC accepts a reasonable excuse.
Missing these deadlines might result in penalties and interest costs, so prepare your documents well in advance of these important dates.
What documents do I need to file my company tax return using CT600?
To ensure your CT600 return is accurate and complete, you must gather several essential documents. Here’s a list of the crucial documents you will require:
Annual Accounts (Financial Statements): You must submit documents such as a balance sheet, profit and loss account and, where required, a cash flow statement. These documents show your company’s financial position and help calculate your taxable profits.
Accounting Period Information: Make sure you have the exact dates for your accounting period, as the CT600 return will be based on these dates. .These usually match your financial year.
Corporation Tax Computation: To determine your taxable profits and tax payable, you will need to use a Corporation Tax computation. This document summarizes your company’s income, authorised allowable expenses, and tax adjustments. HMRC requires it in iXBRL format.
Details of Taxable Income: Include any income or profits your company generated during the accounting period. This may include sales revenue, interest collected, and investment income.
Unique Taxpayer Reference (UTR): You will need your company’s 10 digit Unique Taxpayer Reference (UTR). HMRC uses it to identify your company.
Tax Reliefs & Credits: If your company qualifies for any tax breaks (such as R&D tax relief or losses carried forward), you must provide the relevant supporting documentation to claim them.
iXBRL Tagged Accounts and Computation: HMRC requires both attachments in iXBRL format. Your software normally generates the tagging.
How to File Your Company Tax Return: Step by Step
Filing your UK Company Tax Return (CT600) does not have to be difficult, if you follow the steps in the proper order. Here’s a simple, step by step guide to help you file correctly and stay compliant with HMRC.
- Prepare Your Statutory Accounts: Start with completing your company’s annual accounts. This includes the profit and loss statement, balance sheet, and supporting notes, all prepared according to the guidelines of UK GAAP or IFRS standards. These figures form the basis of your Corporation Tax calculation, so accuracy is crucial.
- Calculate Your Corporation Tax Liability: Adjust your accounting profit for disallowable expenses, capital allowances, and any reliefs (such as R&D or loss relief). This calculates your taxable profit as well as the amount of Corporation Tax owed. The rates are 19% on profits up to £50,000 and 25% on profits above £250,000, with marginal relief in between.
- Choose HMRC Recognised Software: The free HMRC filing service has closed, so use commercial Corporation Tax software or an accountant. Sign in with your Government Gateway details and your company UTR.
- Complete the CT600 Form: The CT600 form is used to record your company’s income, profits, tax calculations, and relief applications to HMRC. It must be completed with HMRC recognised software; paper returns are accepted only in limited cases.
- Attach Supporting Documents: Along with the CT600, please upload your complete statutory accounts and thorough tax computations. HMRC requires these documents to be fully aligned with the data in the return.
- File the Return Online with HMRC: Submit the CT600 and attachments electronically using suitable software. HMRC will send you confirmation once you have submitted your form; retain it for your records.
- Pay Corporation Tax on Time: Corporation Tax is typically payable 9 months and 1 day after the end of your accounting period, while the CT600 can be filed up to 12 months after year end. Missing a payment deadline might result in interest charges.
- Retain Records for Compliance: HMRC requires businesses to maintain accounting and tax records for at least six years. These may be required during compliance inspections or inquiries.
Common Mistakes to Avoid When Filing Your UK Company Tax Return
When filing a company tax return you need to be very careful with every minute detail. Otherwise, it can lead to HMRC enquiries, penalties or delayed refunds. Below are the most common mistakes UK companies make, and how to avoid them.
- Missing the Filing or Payment Deadlines: Many businesses assumes filing and payment deadlines are the same, but they are not. Your Corporation Tax payment is due 9 months and 1 day after the end of your accounting period, whereas the CT600 can be filed up to 12 months after the end of the year. Interest is charged from the day after the payment due date.
- Using Incorrect or Incomplete Financial Figures: Submitting draft or unreconciled accounts is a regular concern. Your CT600 should precisely match your final statutory accounts. Any discrepancy between the accounts, computations, and return raises a red flag for HMRC.
- Claiming Disallowable Expenses: Not every business expense is tax deductible. Client entertainment, personal spending, and some fines or penalties are examples of common disallowed costs. Incorrect claims might overstate reliefs and result in HMRC investigations.
- Forgetting Capital Allowances: Some businesses either ignore capital allowance claims entirely or apply them wrongly. Assets such as equipment, vehicles, and IT acquisitions must be carefully handled to avoid overpaying or underpaying Corporation Tax.
- Submitting Without a Final Review: Rushing to file without a thorough review raises the likelihood of minor but costly mistakes such as improper accounting periods, incorrect company details, or missing attachments.
- Poor Record-Keeping: Incomplete or disorganised records make it difficult to justify figures in the event that HMRC queries them. Companies must keep their tax and accounting records for at least six years.
- Assuming Free HMRC Filing Is Still Available: Since April 2026 you need commercial software to file the CT600, so set it up well before your deadline.
Late Filing Penalties and Interest in 2026
Fixed penalties doubled for returns with a filing deadline on or after 1 April 2026. They apply even if no tax is due.
| Delay | Penalty |
|---|---|
| 1 day late | £200 |
| 3 months late | A further £200 (£400 in total) |
| 6 months late | 10% of unpaid tax, based on HMRC’s estimate |
| 12 months late | A further 10% of unpaid tax |
| 3 returns in a row late | Fixed penalties rise to £1,000 each |
Corporation Tax paid late also accrues interest, which was 7.75% from 9 January 2026.
What Changed for CT600 Filing in 2026
Getting your figures correct is only one aspect of filing your Company Tax Return (CT600) in 2026; another is adhering to HMRC’s most recent compliance requirements. Delays, HMRC inquiries, or fines may result from even minor rule modifications (or neglected digital requirements).
Here are the key 2026 considerations you should keep in mind before submitting:
- Penalties have doubled: From 1 April 2026 a return filed one day late costs £200, and £400 in total at three months.
- The free filing service has closed: HMRC and Companies House closed the joint online service on 31 March 2026, so commercial software is now required.
- Corporation Tax rates still depend on your profit level: Some businesses still fall into various Corporation Tax bands in 2026 based on their taxable profits. The small profits rate is 19% up to £50,000 and the main rate is 25% above £250,000. This means that calculations need to be precise, particularly when marginal relief is involved.
- Businesses are pushed toward digital-first compliance: HMRC is continuing its move toward a fully digital tax system, which means: stricter formatting for returns; increased data checks; higher attention on accuracy of submissions
- CT600 Compliance: 2026 HMRC Focus. In 2026, HMRC is especially sensitive to “avoidable” issues such as: incorrect UTR or accounting period dates; mismatch between accounts and tax computations; missing iXBRL tagging in statutory accounts; incorrect loss relief claims; late submission with no reasonable excuse
- MTD: Key to Corporation Tax Readiness: Corporation Tax is not currently part of Making Tax Digital, but clear digital records are still expected. This impacts how you prepare your CT600 because HMRC expects: clear bookkeeping records; supporting evidence for deductions; clean reconciliation of VAT, payroll, and expenses
- Higher Compliance Focus on Expense Claims: HMRC continues to monitor: director’s loan accounts; travel and subsistence claims; home office claims; salary vs dividend balance; mixed-use expenses
Filing Extensions and Special Circumstances
HMRC does not grant extensions to file your CT600. You have 12 months from the end of your accounting period to file your return.
However, HMRC may waive penalties if you have a valid “reasonable excuse”, such as:
- serious illness or hospitalisation
- bereavement
- fire, flood, theft, or loss of records
- major IT/system failure beyond your control
To avoid interest and penalties, file as soon as you can if you are unable to do so on time, save any supporting documentation, and, if at all feasible, pay the Corporation Tax on time. You must file the return before you can appeal a late filing penalty.
FAQs: Frequently Asked Questions
How do I file my Company Tax Return?
You must use HMRC recognised Corporation Tax software to file your Company Tax Return (CT600) online with HMRC. Your final accounts, tax computations, and any supporting documentation (such iXBRL-tagged accounts) are required. After everything has been examined, you can electronically submit the CT600 and save the confirmation for your records.
When is the deadline for submitting a Company Tax Return and paying Corporation Tax?
Company Tax Return (CT600) Deadline: Within a year following the conclusion of your company’s accounting period, you must submit your CT600 to HMRC.
Deadline for paying Corporation Tax: You have nine months and one day from the end of the same accounting period to pay Corporation Tax. For a 31 March 2026 year end, that means paying by 1 January 2027 and filing by 31 March 2027.
Should I include overseas profits on my Company Tax Return?
Yes, if your UK company is subject to UK Corporation Tax on that income, you are often required to include foreign profits on your Company Tax Return (CT600).
Tax treatment, however, is contingent upon a number of criteria, including the source of the revenue, the tax residence of your business, and the applicability of double taxation relief. To prevent mistakes or overpaying taxes, it is best to take advice from an accountant.
How do I fill out the CT600 form for my company?
You must enter your company’s information, the dates of the accounting period, and the important numbers from your statutory accounts and Corporation Tax computation (income, expenses, allowances, profits, and tax payable) in order to complete the CT600.
The majority of businesses use HMRC recognised Corporation Tax software to complete the CT600, which provides step-by-step instructions and helps minimise errors prior to submission.
Do I still need to submit a tax return if my company made a loss?
Yes. Even if the company incurred a loss and owes no Corporation Tax, you must still file your Corporation Tax Return (CT600) with HM Revenue & Customs. HMRC needs a tax return for each accounting period while the company is operational. A loss does not remove the filing obligation.
What is the penalty for filing a company tax return late?
From 1 April 2026 it is £200 for one day late and a further £200 at three months, followed by 10% tax geared penalties at six and twelve months.
Can I file a company tax return on paper?
Only in limited cases, such as filing in Welsh or where HMRC accepts a reasonable excuse. Most companies must use software.
Long Accounting Periods, First Returns and Amending a Return
If your first accounting period is longer than 12 months, you file two returns, one for the first 12 months and one for the remainder. You can amend a filed return within 12 months of its filing deadline.
Should You File Your Own CT600 or Use an Accountant?
Directors can file for themselves with HMRC recognised software when affairs are simple. An accountant is worth considering for losses, R&D claims, overseas income, capital allowances or a first return, where mistakes are costly.
Conclusion
It doesn’t have to be difficult to file your Company Tax Return (CT600) in 2026, but it does require timely filing and accuracy. You can maintain compliance and steer clear of needless fines or inquiries with the correct documentation, accurate tax calculations, and HMRC recognised filing software.
Working with a qualified accountant or Corporation Tax specialist helps streamline the process if you want to cut down on errors and save time, particularly when handling intricate areas like offshore income, allowances, and year-end adjustments.
Disclaimer: Kindly note this blog provides general information and should not be considered financial advice. We recommend consulting a qualified financial advisor for personalised guidance. We are not responsible for any actions taken based on this content.
