What is CT600: How to File Corporation Tax?

The CT600 Tax Return is the mandatory UK Corporation Tax filing used by limited companies to report taxable profits, claim allowable reliefs, and calculate Corporation Tax owed to HM Revenue & Customs (HMRC) for each accounting period.

The CT600 form is submitted alongside statutory accounts and detailed Corporation Tax computations prepared in iXBRL format, linking financial statements directly to tax reporting requirements.

UK companies must file the CT600 online using HMRC-approved commercial software, as the government’s free online filing service is being withdrawn.

The CT600 filing deadline is 12 months after the end of the accounting period, while Corporation Tax payment is due earlier at 9 months and 1 day after the period end, creating a critical compliance gap.

Late CT600 submissions or overdue Corporation Tax payments trigger automatic penalties, interest charges, and potential HMRC compliance reviews.

What Is a CT600 Tax Return?

The CT600, officially titled the Company Tax Return, is the form prescribed by HM Revenue & Customs (HMRC) for limited companies to declare their income, expenditure, profits, and chargeable gains for a given accounting period.

It is the core mechanism by which HMRC assesses and collects Corporation Tax.

For many, the term ‘Corporation Tax Return’ and CT600 Tax Return are used interchangeably, but the CT600 is simply the physical or digital container for the full submission, which must also include detailed statutory accounts and tax computations. The process of preparing and submitting these documents is mandatory for nearly all UK limited entities.

Purpose of the CT600 Form

The primary function of the CT600 is manifold, serving both the company and the tax authority:

1. Declaration of Taxable Profit: It provides a detailed breakdown of a company’s financial activity, translating accounting profit (based on GAAP or FRS standards) into taxable profit (based on tax legislation).

2. Self-Assessment: The form requires the company to ‘self-assess’ its own Corporation Tax liability, which is a fundamental principle of the UK’s tax system.

3. Claiming Reliefs and Allowances: The form includes sections for claiming essential reliefs, such as Capital Allowances, Research and Development (R&D) Tax Reliefs (via supplementary page CT600L), and losses carried forward or back.

4. Confirmation of Compliance: Submission confirms to HMRC that the company has met its statutory obligation for the accounting period in question.

What Is Corporation Tax and How CT600 Relates to It

Corporation Tax (CT) is a tax levied on the profits of limited companies, registered mutual societies, and other corporate bodies. These profits include:

• Trading profits.

• Investment income (e.g., interest, rents, and non-exempt dividends).

• Chargeable gains from selling assets (e.g., property or shares).

The CT600 is the tool used to calculate this tax. The relationship is direct:

Crucially, the question Is Corporation Tax always 25%? must be addressed. The answer is definitively no. Since 1 April 2023, the UK operates a tiered Corporation Tax system:

Taxable Profit Level (Annual)	Corporation Tax Rate	Notes
Up to £50,000	19% (Small Profits Rate)	Applied to companies with profits below this threshold (pro-rated for short accounting periods or associated companies).
£50,001 to £250,000	Marginal Relief Rate (Tapered)	The rate rises smoothly between 19% and 25%.
Over £250,000	25% (Main Rate)	Applied to companies with profits exceeding this limit (also subject to pro-rating and associated companies rules).

This complexity highlights why accurate preparation of the CT600 is vital to ensure your company pays the correct, and lowest possible, rate.

Is CT600 Mandatory for All UK Companies?

Yes, with very few exceptions. Once a company is incorporated at Companies House, it falls within the scope of Corporation Tax.

The general rule is: Every company that receives a ‘Notice to deliver a Company Tax Return’ from HMRC must file a CT600.

Even if your company has no tax to pay (e.g., it is loss-making or dormant), the filing obligation remains unless HMRC has formally agreed that a return is not required.

Companies that have not been issued a notice but have taxable profits must proactively notify HMRC of their chargeability within 12 months of the end of the accounting period.

Who Needs to File a CT600 Tax Return?

The legal requirement to submit a CT600 Tax Return extends far beyond simply profitable trading businesses. It applies to a broad range of incorporated entities.

Limited Companies

All standard limited companies (Ltd) registered under the Companies Act 2006 must file a CT600 for every accounting period.

This includes companies limited by guarantee, unlimited companies with share capital, and Societas Europaea (SE).

It is a statutory obligation that commences from the date of incorporation, not the date trading begins.

Dormant Companies

A common misconception is that a dormant company is exempt from all filing requirements. This is only partially true.

• Dormant for Companies House: You still need to file dormant accounts with Companies House.

• Dormant for Corporation Tax: If a company is truly dormant (i.e., has no income, no expenses, and no chargeable gains), it should notify HMRC. In such cases, HMRC may agree that the company does not need to submit a CT600. If, however, HMRC issues a Notice to deliver, you must either file a ‘nil’ return or appeal the notice. If you fail to communicate with HMRC, penalties will still be issued.

Newly Registered Companies

New companies face unique deadlines. While the filing deadline for the CT600 remains 12 months after the accounting period end, the first accounting period often lasts longer than 12 months (up to 18 months from incorporation).

If the first set of accounts covers a period longer than 12 months, the company must prepare two separate CT600 Tax Returns.

The first return covers the first 12 months, and the second covers the remainder of the period. This is because a Corporation Tax accounting period (CTAP) cannot exceed 12 months.

Non-Trading or Loss-Making Companies

If your company is non-trading but still receives income (e.g., bank interest or rental income) or incurs expenditure (e.g., administrative costs) or if it is purely loss-making, it still needs to submit a full HMRC CT600.

Submitting the return is critical for loss-making companies as it establishes the quantum of losses that can be carried forward or claimed against prior period profits (or against total profits, depending on the nature of the loss).

Failing to file means failing to declare losses, which sacrifices valuable future tax relief.

You can read more articles on different taxe codes in the UK:

1263L Tax Code: Really a Hidden Gem in your Paycheck
1257L Tax Code: What Does it Mean in Your Paycheck?
D0 Tax Code:HRMC’s Premium Rates Impact on You
500T Tax Code: Guide for UK Tax Payers in 2025
SA302: What is it and How to Get One on 2025?
What is M1 Tax Code: Let’s Decode the UK’s Tax Puzzle

What Information Is Required for CT600?

The CT600 form is highly structured and requires a comprehensive array of financial and corporate data.

Preparing this information correctly is the most time-consuming part of the process and requires the skills of an experienced professional to ensure compliance and tax efficiency.

Company Details and Accounting Period

The initial sections of the form are administrative but vital:

• Company Name and Registration Number: As registered at Companies House.

• Unique Taxpayer Reference (UTR): The company’s specific 10-digit tax ID, issued by HMRC after incorporation.

• Period of Return (Boxes 30 & 35): The start and end dates of the Corporation Tax accounting period. As noted, this period cannot exceed 12 months.

Income, Expenses, and Profit

This is the core financial data section (often boxes 145 to 235). It involves reporting the figures from your statutory accounts, but critically, it demands tax-specific adjustments:

• Trading Profits/Losses: The profit/loss figure before tax adjustments.

• Non-Trading Income: Including interest received, property rental profits, and non-exempt foreign dividends.

• Chargeable Gains: Profits made from the disposal of capital assets, such as land or investment properties.

The most complex part here is calculating the ‘Taxable Profit.’ This involves adding back ‘disallowable expenses’ (e.g., depreciation, business entertaining, and non-wholly-and-exclusively incurred costs) and deducting ‘allowable reliefs’ (e.g., Capital Allowances, detailed below).

This calculation is contained within the supporting tax computations, not the CT600 form itself, though the final figures are transcribed onto the form.

Corporation Tax Calculations

Once the taxable profit is determined, the CT600 guides the company through applying the correct tax rate, taking into account the number of ‘associated companies’ (which reduces the profit thresholds for the small profits rate).

This section reports:

• The calculation of marginal relief, if applicable.

• The final Corporation Tax liability payable (or repayment due).

• Any tax payable on loans to directors (CT600A supplementary page).

Capital Allowances and Reliefs

A key function of the CT600 is claiming reliefs that legitimately reduce the taxable profit. This is where strategic tax planning occurs.

• Capital Allowances (CAs): Unlike depreciation in the accounts, CAs are the HMRC-approved way of deducting the cost of business assets (e.g., equipment, machinery, vans) from profits. The Annual Investment Allowance (AIA) allows a 100% deduction for most qualifying plant and machinery up to a generous annual limit.

• R&D Tax Relief: Small and medium-sized enterprises (SMEs) can claim generous R&D relief against qualifying expenditure, often resulting in a tax deduction or a payable tax credit. This requires the completion of supplementary page CT600L.

• Loss Reliefs: Claiming relief for trading losses by carrying them forward, carrying them back, or setting them against total profits.

Attachments (iXBRL Accounts & Computations)

For almost all online submissions, the CT600 must be accompanied by two mandatory attachments:

1. Statutory Accounts: The company’s annual financial statements (Balance Sheet, Profit & Loss Account, etc.).

2. Tax Computations: The detailed breakdown showing the transition from the accounting profit to the final taxable profit.

Both documents must be submitted in the iXBRL format. iXBRL (Inline eXtensible Business Reporting Language) is a standard that uses electronic “tags” to mark up data items (e.g., ‘Turnover,’ ‘Stock,’ ‘Directors’ Remuneration’) so that HMRC’s systems can read and process the figures digitally.

This technical requirement is why using commercial, HMRC-approved software is almost always necessary, especially if you wish to file the accounts to Companies House and HMRC simultaneously.

How to Complete the CT600 Tax Return

The complexity of the form and the surrounding documentation means that the completion process must be methodical and precise.

Step-by-Step CT600 Completion Process

While software handles the final electronic submission, the manual work remains:

1. Finalise Statutory Accounts: The company’s annual accounts must be prepared and approved by the directors.

2. Calculate Taxable Adjustments: This involves identifying all items that are treated differently for tax purposes than for accounting purposes (e.g., disallowable expenses, non-allowable depreciation).

3. Determine Capital Allowances: Calculate all eligible Capital Allowances, ensuring the company claims the maximum available relief (like the AIA).

4. Draft Tax Computations: A narrative document showing the profit per the accounts, the adjustments made (Step 2), the reliefs claimed (Step 3), and the resultant taxable profit.

5. Complete the CT600 Form: Manually input the final figures (e.g., net taxable profits, reliefs, tax due) from the computations onto the relevant boxes of the CT600 form (or into the software equivalent).

6. Convert to iXBRL: Convert both the accounts and the computations into the mandated iXBRL format.

7. Final Submission: Use HMRC-approved software to bundle the CT600, the iXBRL accounts, and iXBRL computations and submit the entire package online.

Using Accounting Software vs Manual Preparation

The law requires online filing for almost all CT600 submissions.

• HMRC’s Free Online Service: HMRC offers a free service, but it is typically only suitable for companies with very simple affairs and small turnover, and it is scheduled to close in March 2026. Furthermore, it often requires companies to file their accounts separately to Companies House, adding administrative burden.

• Commercial Accounting Software: This is the standard, professional route. HMRC-approved software can simultaneously:

  • Generate the accounts (often integrated with bookkeeping data).

  • Perform the tax adjustments and calculations.

  • Automatically generate and tag the iXBRL files.

  • Submit the CT600 to HMRC and the accounts to Companies House in one seamless process (known as “joint filing”).

For any growing or even moderately complex SME, commercial software (or an accountant using it) is the only practical solution to ensure compliance with the mandatory iXBRL format.

Common Errors to Avoid

As experts in the field, we frequently encounter mistakes that lead to costly penalties or unnecessary tax payments:

Category	Common Error	Impact
Deadlines	Mixing up the payment and filing deadlines.	Late payment interest accrues, and late filing penalties are issued.
Allowances	Failing to claim the Annual Investment Allowance (AIA) or R&D relief.	Overpaying Corporation Tax unnecessarily.
Expenses	Claiming disallowable expenses (e.g., directors’ private expenses, client entertaining) as legitimate business costs.	Inaccurate return penalties (Schedule 24 FA07) and potentially an HMRC enquiry.
iXBRL	Submitting non-iXBRL files, incorrect tagging, or mismatched figures between accounts and CT600.	Submission rejection by HMRC’s systems, resulting in missed deadlines and penalties.
New Companies	Only filing one CT600 for a long first accounting period (over 12 months).	Filing incomplete returns, leading to tax determination by HMRC.

How to Submit a CT600 to HMRC

The process for submitting the CT600 Tax Return has become highly digitised, emphasising speed, accuracy, and adherence to specific digital standards.

Online Submission Requirements

HMRC mandates that virtually all companies must submit their CT600 electronically, a requirement that came into force in 2011. The submission must include:

1. The completed CT600 form.

2. The company’s annual accounts in iXBRL format.

3. The tax computations in iXBRL format.

The only exceptions allowing paper filing are extremely limited (e.g., inability to file online due to physical disability or specific religious beliefs). Simply finding the iXBRL requirement too complicated is not a reasonable excuse, which is why most companies must use approved software or a professional agent.

HMRC-Approved Software Explained

HMRC maintains a list of recognised third-party software providers that meet the technical specifications for electronic filing and iXBRL tagging.

This software is essential because it is designed to communicate directly with the Government Gateway for the submission.

The impending closure of HMRC’s own free online service (scheduled for March 2026) means that commercial software or the use of a tax agent will soon become the only method for CT600 online submission.

This shift underscores the need for directors to either invest in robust software or engage professional support.

Can You File CT600 Without an Accountant?

Legally: Yes. You can file your own CT600, provided you use HMRC-approved software and can produce the mandatory iXBRL accounts and computations.

Practically: It Depends. For a simple, small company with minimal transactions and straightforward finances, self-filing is achievable.

However, if your company has any of the following complexities, engaging an accountant is highly recommended for accuracy, compliance, and tax efficiency:

• Claims for Marginal Relief (due to profits between £50k and £250k).

• Claims for R&D Tax Relief (requiring complex calculation and reporting).

• Foreign income or assets.

• Chargeable capital gains or complex investment transactions.

• Losses to be claimed or carried forward.

• Multiple associated companies.

An accountant not only ensures compliance and avoids penalties but also works strategically to optimise tax liability, potentially saving significantly more than their fee.

CT600 Deadlines You Must Not Miss

Adhering to strict deadlines is arguably the most critical aspect of Corporation Tax compliance. UK tax legislation separates the filing deadline from the payment deadline.

CT600 Filing Deadline

The final deadline for submitting the full CT600 Tax Return, including the accounts and computations, is:

12 months after the end of the accounting period.

If the accounting period ends on 31 December 2024, the filing deadline is 31 December 2025.

Corporation Tax Payment Deadline

The deadline for paying the Corporation Tax owed is significantly earlier than the filing date:

9 months and 1 day after the end of the accounting period.

This discrepancy is a major reason why companies fall foul of HMRC penalties. You must pay the tax bill almost three months before you are required to submit the form detailing the calculation.

First-Year Company Deadlines Explained

For a company’s first accounting period, which may run up to 18 months from the date of incorporation:

Obligation	Deadline	Example: Company incorporated 1 April 2024; Year End set for 31 March 2025 (12 months)
Notify HMRC of chargeability	12 months from AP end	31 March 2026
Corporation Tax Payment	9 months + 1 day from AP end	1 January 2026
CT600 Filing	12 months from AP end	31 March 2026

Deadline Table Example: 31 March Year End

Financial Year End	CT Payment Due Date	CT600 Filing Due Date
31 March 2025	1 January 2026	31 March 2026
31 December 2025	1 October 2026	31 December 2026

Note: Large companies (profits over £1.5 million) must pay tax in quarterly instalments, which have much earlier deadlines.

Penalties for Late or Incorrect CT600 Filing

HMRC imposes stringent penalties to enforce compliance. These are tiered, increasing sharply the longer the delay persists, and they apply even if your company has no tax to pay.

Late Filing Penalties

Penalties for a late CT600 deadline are fixed amounts initially:

Lateness of Filing	Flat-Rate Penalty	Notes
1 day late	£100	Automatic.
3 months late	Additional £100 (Total £200)	Automatic.
6 months late	10% of the unpaid Corporation Tax	Tax-geared penalty. HMRC may issue a ‘Tax Determination’ (estimate).
12 months late	Additional 10% of any unpaid tax	Total tax-geared penalty of 20%.

Escalated Penalties: If the company is late filing three or more returns in a row, the initial £100 penalties jump to £500 each, making the financial consequences severely punitive.

Interest and Surcharges on Late Tax Payments

While there are no fixed penalties for late payment of Corporation Tax in the same way as for late filing (unless you are a large company paying by instalments), HMRC automatically charges interest on any unpaid tax from the day after the due date (9 months and 1 day).

This late payment interest rate (which is the Bank of England base rate plus 2.5%) is calculated daily, making even short delays costly.

HMRC Compliance Checks and Enquiries

HMRC also levies penalties for inaccuracies within the CT600, often found during compliance checks or tax enquiries. These penalties are based on the concept of ‘Potential Lost Revenue’ (PLR) and the behaviour of the company:

Behaviour	Unprompted Disclosure Penalty Range	Prompted Disclosure Penalty Range
Careless (lack of reasonable care)	0% – 30% of PLR	15% – 30% of PLR
Deliberate but not concealed	20% – 70% of PLR	35% – 70% of PLR
Deliberate and concealed	30% – 100% of PLR	50% – 100% of PLR

Source: Corporation Tax: penalties – GOV.UK

Making an unprompted disclosure (telling HMRC about an error before they find it) is the key to mitigating penalties.

How to Amend or Correct a CT600 Tax Return

Perfection is difficult in tax affairs. Fortunately, the system allows for the correction of errors through an amendment process, provided it is done within the statutory time limits.

Amending CT600 Before the Deadline

If you notice an error before the 12-month filing deadline, you can simply submit a corrected return via your filing software. The later version automatically replaces the earlier one.

Correcting Errors After Submission

The statutory time limit for correcting or amending your CT600 is:

12 months from the statutory filing deadline.

In essence, you have 24 months from the end of the accounting period to amend your return. This correction is typically done by resubmitting an amended CT600 and updated iXBRL attachments through the same method used for the original filing.

This time limit is critical for claiming reliefs or losses discovered after the initial filing.

After this window closes, amendments can only be made in exceptional circumstances or via a formal claim for ‘Error or Mistake Relief’ under specific legislation.

What to Do If HMRC Contacts You

If HMRC initiates a compliance check or tax enquiry, the most important rule is to engage promptly and professionally.

1. Do Not Ignore It: Ignoring HMRC correspondence will only escalate the matter and increase potential penalties.

2. Seek Professional Advice: Immediately contact your tax adviser or accountant. They can manage all correspondence, understand the specific scope of the enquiry, and negotiate disclosures.

3. Co-operate: Co-operation with HMRC during an enquiry—providing information, making records accessible—is a key factor in mitigating any penalties that may arise from errors found.

CT600 Tax Return for Special Situations

Certain company types or circumstances require the use of supplementary pages or have modified filing requirements.

CT600 for Loss-Making Companies

As mentioned, a loss-making company must still file a CT600 Tax Return to formally establish the loss. This is crucial for future tax relief. Key considerations include:

• Carrying Back Losses: Trading losses can be carried back to offset profits made in previous accounting periods, potentially generating a tax repayment.

• Group Relief: If the company is part of a corporate group, losses may be surrendered to offset profits of other group members (using supplementary page CT600C).

CT600 for Dormant Companies

A company that is dormant for Corporation Tax purposes may receive notification from HMRC stating that a CT600 is not required.

However, if they do receive a Notice to deliver, they must submit a ‘nil’ return (a CT600 with all relevant boxes showing zero).

Failing to file a nil return when requested still triggers the £100 late filing penalty. The safest course is always to inform HMRC of dormancy and retain confirmation of non-requirement.

Closing a Company and Final CT600

When a company ceases trading and is preparing for dissolution (being struck off the register), a final CT600 must be filed.

This return will cover the period from the previous accounting period end up to the date trading ceased.

The company must settle all its tax liabilities, including the final Corporation Tax payment, before formally applying to Companies House for dissolution. This final return confirms that all tax affairs have been wrapped up.

CT600 vs Other HMRC Tax Returns

To solidify the understanding of the CT600’s unique position, it is helpful to compare it with the other common tax returns in the UK.

CT600 vs SA100 (Self Assessment)

Feature	CT600 (Company Tax Return)	SA100 (Self Assessment)
Who Files?	UK Limited Companies, associations, clubs, and foreign companies with a UK branch.	Sole traders, partnerships, high-income individuals, landlords, and company directors.
Tax Paid	Corporation Tax on company profits.	Income Tax and National Insurance Contributions (NICs) on personal income/profits.
Format	CT600 form + iXBRL accounts + computations.	SA100 main form + supplementary pages (e.g., SA103 for self-employment).
Legal Basis	Corporation Tax Act 2009.	Income Tax Act 2007.
	A director may need to file both: a CT600 for the company and an SA100 for their personal salary/dividend income.	The semantic keyword “CT600 vs SA100” is explicitly addressed.

CT600 vs VAT Return

A Value Added Tax (VAT) Return is a transactional tax return, completely separate from the profit-based CT600.

• VAT Return: Filed typically quarterly, detailing output VAT (charged to customers) minus input VAT (paid on purchases). It is a declaration of collected tax, not a tax on the business’s profit.

• CT600: Filed annually, detailing the company’s net profit (which is calculated after the impact of VAT has been removed) and the resulting Corporation Tax due.

CT600 vs PAYE Submissions

The Pay As You Earn (PAYE) system manages tax and NICs deducted from employees’ salaries via Real Time Information (RTI) submissions (Full Payment Submissions/FPS).

• PAYE/RTI: Submitted monthly or upon payment of wages, managing tax withheld at source.

• CT600: The salary and employer NICs paid through the PAYE system are recorded as allowable expenses within the company’s accounts, thus reducing the profit subject to Corporation Tax calculation in the CT600.

FAQs About CT600 Tax Return

 

What happens if I don’t file CT600?

If you fail to file your CT600 by the deadline (12 months after the accounting period end), HMRC will impose automatic, tiered late-filing penalties starting at £100. If the delay extends past six months, penalties become tax-geared, adding 10% of the unpaid Corporation Tax at six months and another 10% at twelve months.

Can I file CT600 myself?

Yes, you can file the CT600 yourself. However, you must use HMRC-approved software to submit the Company Tax Return form and the required accompanying documents (statutory accounts and tax computations) in the mandatory iXBRL digital format.

Do small companies need to file CT600?

Yes. The obligation to file a CT600 Tax Return applies to all limited companies, regardless of size, turnover, or whether they made a profit or a loss. The only way to be exempt is if HMRC formally agrees that your company is dormant for Corporation Tax purposes and waives the requirement to file.

How long does HMRC take to process CT600?

The electronic processing of the CT600 is usually swift, often within a few weeks, especially if the return is accurate and filed via commercial software. However, if the return includes complex claims (e.g., R&D Tax Credits) or results in a tax repayment, the processing time can extend to several months as HMRC will need to conduct additional checks before making a payment.

The Bottom Line

The CT600 Tax Return process is foundational to operating a UK limited company. By maintaining a firm understanding of the requirements, you can ensure full compliance and avoid unnecessary penalties, protecting your business’s financial health.

While the requirements can appear daunting, thousands of UK companies successfully navigate this process every year.

With meticulous record-keeping, the use of up-to-date, HMRC-approved accounting software, and strategic input from a qualified UK Tax Advisor, compliance is entirely achievable.

The content provided on TaxCalculatorsUK, including our blog and articles, is for general informational purposes only and does not constitute financial, accounting, or legal advice.

You can also visit HMRC’s official website for more in-depth information about the topic.