What Is a CT600 Corporation Tax Return?
What Is a CT600 Corporation Tax Return?
Understand what a CT600 is, which companies must submit one, what information HMRC requires and how to avoid missing your Corporation Tax deadlines.
Prepared by The Online Accountants · ACCA qualified expertise · UK limited company specialists
What is a CT600 Corporation Tax Return?
A CT600 is the main Corporation Tax Return form used by UK companies to report their taxable profits, losses, reliefs and Corporation Tax liability to HMRC.
The CT600 forms part of a company's annual tax reporting. A complete Company Tax Return normally comprises the CT600 form, supporting tax computations and statutory accounts prepared in the required electronic format.
It allows HMRC to understand how a company has calculated the Corporation Tax it owes, including any adjustments made to its accounting profit.
CT600 form
Reports the company's taxable results, tax calculations, reliefs and Corporation Tax payable.
Tax computations
Explain how accounting profits are adjusted to arrive at taxable profits or allowable losses.
Company accounts
Provide the financial statements supporting the tax return, normally submitted to HMRC in iXBRL format.
Who needs to file a CT600?
UK limited companies that are within the charge to Corporation Tax generally need to submit Company Tax Returns when required by HMRC.
Companies commonly required to file
- Trading private limited companies
- Companies making taxable investment or rental income
- Companies reporting trading losses
- Companies claiming Corporation Tax reliefs
- Other organisations within the Corporation Tax regime
Situations needing particular attention
- Newly incorporated companies
- Companies that have stopped trading
- Dormant companies receiving HMRC filing notices
- Companies with accounting periods longer than 12 months
- Companies with no Corporation Tax to pay
What information goes into a CT600?
A CT600 brings together the company's financial results, tax adjustments and claims. The exact information depends on the company's circumstances.
Company information
Company name, registration details, Corporation Tax Unique Taxpayer Reference (UTR) and accounting period.
Income and profits
Trading income, property income, chargeable gains and other relevant taxable income.
Tax adjustments
Disallowable expenditure, capital allowances, depreciation adjustments and other tax differences.
Reliefs and losses
Relevant loss relief, charitable donations, available allowances and qualifying tax reliefs.
Corporation Tax calculation
Taxable profits, applicable rates, marginal relief where relevant, and the final liability.
Supplementary pages
Additional CT600 schedules where required, such as certain loan-to-participator or relief claims.
When is a CT600 due?
A Company Tax Return is normally due 12 months after the end of its Corporation Tax accounting period. Corporation Tax is usually payable earlier, at 9 months and 1 day after that period ends.
CT600 Filing Deadline Calculator
Enter the last day of your Corporation Tax accounting period to calculate the normal deadlines.
This calculator estimates the standard deadlines. Instalment payment rules can apply to larger companies and companies with associated companies. Special filing circumstances and accounting-period changes may affect the result. Check HMRC notices for your actual deadlines.
For more detailed guidance, read When Is Corporation Tax Due?.
How does a CT600 calculate Corporation Tax?
A company's accounting profit is not necessarily the same as its taxable profit. Certain expenses may be disallowed for tax purposes, while capital allowances and other reliefs can reduce the taxable amount.
| Illustrative tax computation | Amount |
|---|---|
| Profit before tax in company accounts | £40,000 |
| Add back depreciation | + £5,000 |
| Add back disallowable expenses | + £1,000 |
| Deduct qualifying capital allowances | − £6,000 |
| Illustrative taxable profits | £40,000 |
| Corporation Tax at 19% | £7,600 |
Simplified example assuming a 12-month accounting period, no associated companies, no other taxable income or adjustments, and eligibility for the 19% small profits rate. Actual computations can be more complex.
What Corporation Tax rates apply?
The main Corporation Tax rate is 25%, while the small profits rate is 19% for qualifying companies with profits up to £50,000. Marginal relief can apply between £50,000 and £250,000.
These thresholds can be reduced for short accounting periods and associated companies. Special rules apply to certain types of companies and income.
See our guide to Limited Company Tax for further explanations.
How to file a CT600 online
Since 1 April 2026, companies generally need commercial software to file their Company Tax Returns electronically. HMRC's former free online filing service closed on 31 March 2026.
Prepare accurate bookkeeping records
Reconcile bank balances, review income and expenses, and identify outstanding invoices and liabilities.
Prepare the statutory accounts
Complete the company's financial statements under the relevant accounting standards.
Calculate taxable profits
Adjust accounting profits for disallowable expenditure, capital allowances, losses and other tax items.
Complete the CT600
Enter the relevant company details, taxable results, reliefs and supplementary information.
Submit through compatible software
File the CT600 with the required iXBRL accounts and computations using suitable commercial software or an authorised accountant.
Check acceptance and arrange payment
Keep the HMRC submission acknowledgement and ensure any Corporation Tax is paid by its separate deadline.
What happens if a CT600 is filed late?
HMRC can charge automatic penalties when a Company Tax Return is submitted after its deadline, even where no Corporation Tax is payable.
| How late? | Normal penalty |
|---|---|
| 1 day late | £200 |
| 3 months late | Another £200 |
| 6 months late | 10% of unpaid Corporation Tax, based on HMRC's determination |
| 12 months late | Another 10% of unpaid Corporation Tax |
If a return is late three times consecutively, the initial fixed penalties can increase to £1,000 each. Late payment interest and separate penalties for inaccurate returns may also apply.
CT600 vs company accounts: what's the difference?
| Requirement | CT600 | Statutory accounts |
|---|---|---|
| Submitted to | HMRC | Companies House (and accounts also accompany the HMRC tax return) |
| Main purpose | Report Corporation Tax liability | Report company financial performance and position |
| Normal deadline | 12 months after Corporation Tax period end | 9 months after financial year end for an established private company |
| Required format | Electronic CT600 and supporting information | Format depends on recipient and filing requirements |
| Prepared by | Company or accountant | Company directors, often assisted by accountants |
First accounts for a newly incorporated private limited company normally have a different Companies House deadline: 21 months after incorporation.
Read our How to File Company Accounts guide for the wider filing process.
Common CT600 mistakes to avoid
Using accounting profit as taxable profit
Tax adjustments are often necessary before calculating the Corporation Tax liability.
Missing capital allowances
Eligible expenditure on business equipment may qualify for valuable tax deductions.
Incorrect accounting periods
A Corporation Tax accounting period cannot exceed 12 months, even where company accounts cover longer.
Overlooking director's loans
Certain outstanding loans to participators can create additional tax charges and reporting.
Forgetting losses and reliefs
Relief claims must be supported, correctly calculated and made within the relevant rules.
Confusing payment and filing
Corporation Tax is normally payable before the CT600 submission deadline arrives.
CT600 Corporation Tax Return FAQs
Is a CT600 the same as a Corporation Tax Return?
The CT600 is the main form within a Company Tax Return. A complete return normally includes the CT600, tax computations and supporting company accounts.
Can I file a CT600 myself?
Yes. A company can prepare and file its own CT600 using suitable commercial software. An accountant can also prepare and submit it on the company's behalf.
Is CT600 filing free?
HMRC's free online filing service closed on 31 March 2026. Commercial software generally involves a cost, although pricing varies. Accountants may charge separately or include filing within a package.
Do I need a CT600 if my company made a loss?
Normally yes, if HMRC has issued a notice requiring a Company Tax Return. A loss does not automatically remove the filing obligation.
Does a dormant company need to file a CT600?
A company recognised as dormant for Corporation Tax may not need to submit ongoing returns. However, if HMRC has issued a filing notice, the company should deal with that notice and obtain confirmation if HMRC withdraws the requirement.
What is the CT600 filing deadline?
The normal deadline is 12 months after the end of the Corporation Tax accounting period. Specific circumstances can affect the deadline.
Can a CT600 cover more than 12 months?
No. A Corporation Tax accounting period cannot exceed 12 months. If company accounts cover a longer period, two Company Tax Returns may be necessary.
Can I amend a CT600 after filing?
Yes. Companies can normally amend a Company Tax Return within 12 months of the statutory filing deadline. Other procedures may apply after that period.
Do I need an accountant to file a CT600?
There is no general legal requirement to appoint an accountant. However, professional assistance can be useful where tax adjustments, director's loans, losses, associated companies or relief claims make the return more complicated.
Related Corporation Tax guides
When Is Corporation Tax Due?
Understand payment deadlines and how they differ from CT600 filing dates.
Read guide →How to Reduce Corporation Tax
Explore legitimate tax reliefs and planning opportunities for limited companies.
Read guide →Corporation Tax Payment Reference
Learn how to identify and use the correct reference when paying HMRC.
Read guide →Need Help Preparing and Filing Your CT600?
The Online Accountants helps UK limited companies prepare statutory accounts, calculate Corporation Tax and submit CT600 returns to HMRC.
Get experienced accounting support, clear pricing and help meeting your company filing obligations.
View Company Accounts Filing → View Accountants FeesGuide updated October 2026. General UK tax information; individual circumstances and legislative changes may affect your obligations.