CT600 Explained: A Complete Guide to the UK Company Tax Return

If you run a UK limited company, you will eventually come across CT600. The name sounds technical, but the concept is straightforward: CT600 is the main form used to submit a UK company's Corporation Tax return to HM Revenue & Customs (HMRC). A CT600 tells HMRC about the company's taxable profits, tax calculations, reliefs and other relevant information for a particular accounting period. It normally forms part of a wider Company Tax Return that also includes the company's accounts and tax computations.

For founders, particularly those operating UK companies from overseas, understanding CT600 is important because filing annual accounts at Companies House does not replace the need to file a Corporation Tax return with HMRC. This guide explains what CT600 means, who needs to file it, what information it contains, when it is due, how Corporation Tax is calculated, what supplementary pages are, and what happens if you file late.

What Is a CT600?

A CT600 is the Company Tax Return form used by a company to report its Corporation Tax position to HMRC. The complete Company Tax Return is broader than the CT600 form itself. HMRC describes it as consisting of:

• The CT600 form
• Any relevant supplementary pages
• The company's accounts
• Tax computations

iXBRL Requirement: The accounts and computations must generally be submitted electronically in iXBRL format. In practical terms, the CT600 connects your company's accounting information with its Corporation Tax calculation.

Example Scenario

Imagine a UK consulting company records:

• £120,000 revenue
• £70,000 allowable business expenses
• £50,000 accounting profit

The company cannot simply type £50,000 into a box and assume that is its final taxable profit. Tax adjustments, capital allowances, losses, reliefs and other rules may affect the Corporation Tax calculation. The CT600 is where the resulting tax position is reported to HMRC.

Is CT600 the Same as Annual Accounts?

No. This is one of the most common mistakes made by new company owners.

• Annual accounts are generally filed with Companies House and report the company's financial performance and financial position.
• CT600 relates to the company's Corporation Tax return and is submitted to HMRC.

The two processes use overlapping financial information, but they serve different purposes.

A company can therefore have both a Companies House accounts deadline and an HMRC Company Tax Return deadline. This distinction is particularly important for international founders who may assume that their accountant's Companies House filing has taken care of everything.

Who Needs to File a CT600?

A company must submit a Company Tax Return when HMRC issues it with a notice to deliver a Company Tax Return. HMRC states that receiving such a notice creates a legal obligation to submit the return. This can apply to companies of many different types and sizes. A company may need to file even when:

• It made little or no profit
• It is newly incorporated
• It has made a loss
• It is temporarily inactive
• Its taxable profit is zero
• It is claiming tax relief
• It has Corporation Tax to pay

Important: Having no Corporation Tax to pay does not necessarily mean there is no filing requirement. If HMRC has issued a notice to deliver a return, the company should deal with it rather than simply assuming that a nil tax bill means no CT600 is necessary.

What Information Is Included on a CT600?

The exact information required depends on the company and its activities, but the CT600 covers several important areas.

1. Company Information

The return identifies the company and the accounting period covered. This includes information such as the company's:

• Name
• Registration details
• Unique Taxpayer Reference (UTR)
• Accounting period

Getting the accounting period correct is essential because Corporation Tax deadlines are tied to the period being reported.

2. Income and Profits

The return captures information relevant to calculating the company's taxable profits. This can involve:

• Trading income
• Other income
• Chargeable gains
• Profits before deductions
• Tax adjustments
• Losses
• Reliefs

Accounting profit and taxable profit are not always identical.

3. Tax Calculation

The CT600 ultimately records the Corporation Tax calculation, including relevant reliefs, reductions and the amount payable or refundable. HMRC's current CT600 guidance specifically covers sections relating to company information, tax calculations, income, chargeable gains, profits, deductions, reliefs, tax reconciliation and other indicators.

How Is Corporation Tax Calculated?

The CT600 reports the tax calculation, but understanding the underlying process helps make the form much easier to understand.

Example

Suppose a small UK company has an accounting profit of £60,000. After necessary tax adjustments, assume its taxable profit remains £60,000. For the 2026 financial year:

• Small profits rate: 19% for companies with profits under £50,000.
• Main rate: 25% for companies with profits over £250,000.
• Marginal relief: Companies with profits between these limits can potentially qualify for marginal relief.

Therefore, a company with £60,000 of taxable profits does not simply fall into the 19% category. The marginal relief rules need to be considered. The actual calculation can also be affected by associated companies and the length of the accounting period. This is one reason why founders should not estimate their tax bill solely by multiplying their accounting profit by 19% or 25%.

What Is Marginal Relief?

Marginal relief applies to companies whose profits fall between the lower and upper limits. For the 2026 financial year, the relevant limits are:

• £50,000: Lower limit
• £250,000: Upper limit
• 19%: Small profits rate
• 25%: Main rate
• 3/200: Standard marginal relief fraction

These limits generally need to be adjusted where the accounting period is shorter or longer than 12 months, and associated companies can also affect the limits. For this reason, the Corporation Tax calculation is more nuanced than the headline rates suggest.

What Are CT600 Supplementary Pages?

Not every company needs supplementary pages. However, certain situations require additional information to accompany the CT600. Depending on the company's circumstances, supplementary pages can include:

• CT600A — Close company loans: Relevant to certain loans or arrangements involving participators in close companies. For example, if a close company makes a loan to a shareholder and the relevant conditions apply, CT600A may be required.
• CT600B — Controlled foreign companies and foreign permanent establishments: Applies where a UK company has relevant interests in foreign companies or certain foreign permanent establishment matters.
• CT600C — Group and consortium relief: Relevant where companies make claims or surrenders involving group or consortium relief.
• CT600D — Insurance: Applies to certain insurance companies, including relevant members of Lloyd's and friendly societies.

HMRC lists several other supplementary pages covering areas such as charities, tonnage tax, cross-border royalties, R&D and certain sector-specific charges. Takeaway: Do not assume every company needs every CT600 supplementary page.

When Is a CT600 Due?

Understanding deadlines is essential, as the payment and filing deadlines are different.

• CT600 Filing Deadline: Generally 12 months after the end of the accounting period covered by the Company Tax Return.
• Corporation Tax Payment Deadline: For most companies, Corporation Tax is normally due 9 months and 1 day after the end of the accounting period.

Example Timeline

Suppose your company's accounting period ends on 31 December 2026:

• Corporation Tax payment due: 1 October 2027
• Company Tax Return (CT600) due: 31 December 2027

Crucial Distinction: You may have to pay the tax before you have to file the CT600. Founders should avoid treating the CT600 deadline as their tax payment deadline.

What Happens If You File CT600 Late?

Late filing can result in penalties. HMRC applies:

• 1 day late: £200 penalty
• 3 months late: Another £200 penalty
• 6 months late: HMRC may estimate the Corporation Tax bill and add a penalty of 10% of unpaid tax
• 12 months late: Another 10% of unpaid tax

If a company files its tax return late three times consecutively, the £200 fixed penalties can increase to £1,000 each. Interest can also apply where Corporation Tax is paid late. The lesson for founders is straightforward: do not wait until you know exactly how much tax is payable before preparing the return.

How Do You File a CT600?

Most companies need to file their Company Tax Return online using appropriate commercial software. HMRC's current guidance says companies and their agents should use commercial software for Corporation Tax filing, with the accounts and computations submitted in iXBRL format. This means the process is generally handled through:

1. The company's accounting records
2. Annual accounts
3. Tax computations
4. Corporation Tax software
5. CT600 and any relevant supplementary pages
6. Electronic submission to HMRC

An accountant or tax adviser can usually handle this process on the company's behalf.

What Documents Do You Need for a CT600?

Preparing early makes the process much easier. Typically, your accountant or tax preparer may need:

• Company accounting records
• Bank statements
• Sales invoices & purchase invoices
• Expense records & payroll information
• Asset and equipment records
• Loan statements & details of dividends
• Director loan information
• Previous tax returns & details of losses brought forward
• Details of tax relief claims
• Annual accounts
• Information about overseas transactions

For an international business, you may also need information about foreign income, overseas subsidiaries, permanent establishments, cross-border transactions and currency conversions.

CT600 for Non-Resident Company Owners

This is especially important for global founders. A founder's personal residence outside the UK does not automatically eliminate a UK company's Corporation Tax obligations. If you establish a UK limited company while living in another country, the company itself can still have UK tax filing obligations.

For example, a founder living in Nigeria may own a UK company providing digital services to customers in Europe. The founder's location does not by itself remove the company's responsibility to deal with HMRC where a Company Tax Return is required. However, international structures can become complicated. Questions may arise around:

• Where the company is tax resident
• Where management and control takes place
• Permanent establishments
• Double taxation agreements
• Transfer pricing
• Foreign income & withholding taxes
• Overseas subsidiaries & controlled foreign companies
• Tax residence of the founder

This is where a standard CT600 filing should not be confused with comprehensive international tax planning. A UK company formation platform such as IncorpUK may help global founders establish and manage a UK company, but company formation itself does not determine the complete international tax position of the founder or business.

CT600 and R&D Tax Relief

Research and development is another area that can complicate the Corporation Tax return. Companies undertaking qualifying R&D may have additional reporting requirements and tax relief considerations. The treatment depends on the company's activities, accounting period, qualifying expenditure and the applicable R&D regime.

Because the rules and filing requirements have changed significantly in recent years, companies claiming R&D relief should ensure their claim is prepared using current HMRC requirements rather than relying on an old template or historical calculation.

CT600 Checklist for UK Companies

Before submitting your Company Tax Return, check off these steps:

• [ ] Correct accounting period confirmed
• [ ] Company UTR available
• [ ] Bank records reconciled
• [ ] Sales and expenses recorded
• [ ] Annual accounts prepared
• [ ] Tax adjustments reviewed
• [ ] Capital expenditure considered
• [ ] Losses checked
• [ ] Tax reliefs reviewed
• [ ] Director loans considered
• [ ] Dividends reviewed
• [ ] Overseas transactions identified
• [ ] Associated companies considered
• [ ] Correct Corporation Tax rate applied
• [ ] Supplementary pages checked
• [ ] Tax payment deadline recorded
• [ ] CT600 filing deadline recorded
• [ ] Submission confirmation retained

Common CT600 Mistakes to Avoid

1. Confusing accounting profit with taxable profit: Your accounts may show a profit, but the figure used for Corporation Tax purposes can require adjustments.
2. Treating the CT600 and Companies House accounts as the same filing: They are separate obligations and should be managed separately.
3. Forgetting the payment deadline: Corporation Tax is generally payable 9 months and 1 day after the accounting period ends, which is earlier than the standard CT600 filing deadline.
4. Ignoring associated companies: Associated companies can affect Corporation Tax thresholds and marginal relief calculations.
5. Assuming a loss means no CT600: A company can still have a filing obligation even when it does not owe Corporation Tax.
6. Leaving international tax questions until filing day: Non-resident founders and companies with cross-border activities should identify international tax issues early.

Frequently Asked Questions

What does CT600 stand for?

CT600 refers to the Company Tax Return form used to report a company's Corporation Tax position to HMRC.

Is CT600 the same as Corporation Tax?

No. Corporation Tax is the tax itself, while CT600 is the form used as part of the process of reporting the company's Corporation Tax position.

Is CT600 the same as annual accounts?

No. Annual accounts are generally filed with Companies House, while the CT600 forms part of the Company Tax Return submitted to HMRC.

When does a CT600 need to be filed?

A Company Tax Return is generally due 12 months after the end of the accounting period to which it relates.

When does Corporation Tax need to be paid?

For most companies, Corporation Tax is normally due 9 months and 1 day after the end of the accounting period.

Can I file a CT600 myself?

It is possible for some companies to handle their own Corporation Tax filing, but the process involves accounts, tax computations and potentially supplementary pages. Many businesses use accounting software or an accountant.

Does a dormant company need a CT600?

Not necessarily in every situation. The requirement depends on whether HMRC has issued a notice to deliver a Company Tax Return and the company's tax status. A company should not simply assume that inactivity removes every HMRC obligation.

Do non-resident owners have to file a CT600?

A non-resident owner can own a UK company that has UK Corporation Tax filing obligations. The owner's personal residence does not automatically remove the company's obligations.

What happens if a CT600 is filed late?

HMRC can impose fixed and tax-related penalties. A return that is one day late can trigger a £200 penalty, with further penalties applying as the delay increases.

Can a CT600 be corrected after filing?

Yes. Companies can generally amend Company Tax Returns within the applicable amendment period. The appropriate process depends on what needs to be corrected and when the original return was submitted.

Conclusion

The CT600 is the central form used to report a company's Corporation Tax position to HMRC, but it should be viewed as part of a larger compliance process rather than an isolated form. For most companies, the process starts with accurate bookkeeping and annual accounts, moves through tax adjustments and relief calculations, and ends with the CT600, supporting computations and any necessary supplementary pages.

The deadlines are equally important. Corporation Tax is generally due 9 months and 1 day after the accounting period ends, while the Company Tax Return is normally due 12 months after the accounting period ends. For founders, especially those operating UK companies from abroad, the biggest takeaway is that incorporation is only the beginning. A UK company creates continuing accounting, tax and reporting responsibilities.

If you keep accurate records throughout the year, distinguish Companies House obligations from HMRC obligations, monitor both the tax payment and CT600 filing deadlines, and obtain professional advice when your business becomes internationally or financially complex, Corporation Tax compliance becomes far more manageable and far less likely to turn into an expensive last-minute problem.