What Is a CT600 Return? A Plain-English Guide for UK Limited Companies

A CT600 is the main form in a UK Company Tax Return. It reports a company’s income, taxable profits or losses, reliefs and Corporation Tax calculation to HMRC.

The complete Company Tax Return package is broader than the form alone. It normally includes the CT600, any relevant supplementary pages, company accounts and the Corporation Tax computation.

Official source: GOV.UK — Company Tax Return obligations.

A company or organisation must file when HMRC issues a notice to deliver a Company Tax Return, usually called a CT603. This can apply to limited companies and certain clubs, societies, associations and other organisations within Corporation Tax.

A dormant company does not automatically need to send HMRC a CT600 every year. If HMRC has issued a notice to file, the company must respond unless HMRC withdraws it. Dormant annual accounts for Companies House are a separate obligation.

Do not invent a Unique Taxpayer Reference. A genuine 10-digit Corporation Tax UTR is required for an HMRC filing; it is not required merely to file dormant accounts with Companies House.

Understanding the deadlines for your company tax return is crucial to avoid penalties. There are two key dates to remember:

Payment Deadline

Corporation tax must be paid to HMRC 9 months and 1 day after the end of your accounting period. For example, if your accounting period ends on 31 March 2026, your tax payment is due by 1 January 2027.

Filing Deadline

Your company tax return must be filed with HMRC 12 months after the end of your accounting period. Using the same example, your filing deadline would be 31 March 2027.

Note that the payment deadline comes before the filing deadline. This means you need to calculate and pay your tax before you're required to submit the full company tax return.

Late Filing Penalties

Miss your company tax return deadline and the penalties stack up quickly:

• 1 day late — £100 penalty
• 3 months late — additional £100 penalty
• 6 months late — HMRC estimates your tax and adds 10% of the unpaid amount
• 12 months late — a further 10% of the unpaid tax

These penalties apply on top of any interest charged on late tax payments. Using reliable software to file your company tax return on time is one of the simplest ways to protect your business from unnecessary costs.

Before you start completing your company tax return, gather the following information:

Company Details

• Company registration number
• Company UTR (Unique Taxpayer Reference) — issued by HMRC when you registered for corporation tax
• Registered office address
• Accounting period start and end dates

Financial Information

• Turnover — total sales revenue for the period
• Cost of sales — direct costs of producing goods or services
• Administrative expenses — overheads including rent, utilities, salaries, and professional fees
• Other income — bank interest, investment income, or other non-trading income
• Capital allowances — tax relief on business assets like equipment, vehicles, or machinery

Balance Sheet

• Fixed assets (tangible and intangible)
• Current assets (cash, debtors, stock)
• Current liabilities (creditors, tax owed, overdrafts)
• Shareholders' funds (share capital, retained earnings)

If you use accounting software like Xero, QuickBooks, or FreeAgent, much of this data can be imported directly into your company tax return filing tool.

Filing your company tax return involves several steps. Here's a clear walkthrough of the process:

Step 1: Prepare Your Accounts

Before filing your company tax return, you need a complete set of accounts for the period. For most small companies, this means a profit and loss statement and a balance sheet.

Step 2: Calculate Your Corporation Tax

Based on your accounts, calculate the corporation tax due. Since April 2023, the rates are:

• 19% on profits up to £50,000 (small profits rate)
• 25% on profits over £250,000 (main rate)
• Marginal relief applies for profits between £50,000 and £250,000

These thresholds are divided by the number of associated companies plus one. Good company tax return software calculates this automatically.

Step 3: Complete the CT600 Form

The CT600 is the official company tax return form. It captures key financial data including turnover, trading profits, investment income, and tax calculations. There are also supplementary pages for specific situations like capital gains or R&D claims.

Step 4: Generate iXBRL Accounts

HMRC requires your accounts to be submitted in iXBRL format alongside the company tax return. This is a machine-readable version of your accounts with standardised financial tags. CT600 software like WeFile generates this automatically.

Step 5: Submit to HMRC

Your company tax return is submitted electronically to HMRC. You'll receive a confirmation with an IRmark (a digital signature) as proof of submission.

Step 6: File Accounts with Companies House

In addition to filing your company tax return with HMRC, you must also file annual accounts with Companies House. Many filing tools, including WeFile, let you submit to both in one go.

Even if your company is dormant — meaning it has had no significant accounting transactions — you still need to file a company tax return. However, the process is much simpler.

A dormant company tax return is essentially a nil return confirming that:

• The company had no turnover
• No corporation tax is due
• The balance sheet shows only the initial share capital (typically £1 or £100)

With WeFile, filing a dormant company tax return takes less than 10 minutes. Simply select the dormant account type, confirm the nil figures, and submit. There's no need to pay an accountant hundreds of pounds for what is fundamentally a blank form.

It's worth noting that after filing your first dormant company tax return, you can sometimes ask HMRC to make your company dormant for CT purposes, which may mean you don't need to file in subsequent years — but this must be confirmed with HMRC directly.

Filing your company tax return incorrectly can lead to HMRC enquiries, penalties, and overpayment of tax. Here are the most common mistakes to watch out for:

1. Using the Wrong Accounting Period

Your company tax return must cover the exact accounting period registered with HMRC. If your accounting period doesn't match, your return will be rejected. Filing software that pulls period data from Companies House eliminates this risk.

2. Forgetting to Claim Allowable Expenses

Many small company directors forget to claim legitimate business expenses on their company tax return, resulting in overpayment of corporation tax. Common overlooked expenses include:

• Use of home as office
• Professional subscriptions
• Business travel and mileage
• Training and professional development

3. Incorrect Capital Allowances

Capital allowances on your company tax return must be calculated correctly. The Annual Investment Allowance (AIA) lets you claim 100% tax relief on qualifying capital expenditure up to £1 million per year.

4. Not Filing Because the Company Is Dormant

A surprisingly common mistake. Many directors don't realise that a dormant company still needs to file a company tax return. Failing to file results in automatic penalties regardless of whether any tax is due.

5. Missing the Balance Sheet

Your company tax return must include a balance sheet that balances (assets = liabilities + equity). An unbalanced balance sheet will cause your submission to be rejected.

The cost depends on the route and the complexity of the company. For a straightforward company whose director can prepare accurate figures, self-filing with suitable commercial software is generally the lowest-cost compliant route. Complex positions may warrant professional advice.

Rather than quote an unsourced market-wide accountant or software benchmark, compare current published supplier prices directly. WeFile’s current live prices are shown on our cheapest way to file a CT600 in 2026 guide.

If you are choosing a provider, see our 2026 CT600 filing software comparison for a feature-by-feature overview of leading options.

WeFile was purpose-built to make filing your company tax return as simple and affordable as possible. Here's what sets us apart:

• HMRC-recognised — WeFile is officially approved for electronic CT600 submission
• Dual filing — submit your company tax return to HMRC and your annual accounts to Companies House in one process
• Automatic calculations — corporation tax, marginal relief, and capital allowances are calculated as you enter your figures
• iXBRL generation — fully compliant accounts generated automatically, no technical knowledge required
• Step-by-step wizard — guided process that makes filing accessible to anyone
• Import from Xero, QuickBooks & FreeAgent — pull your trial balance data directly into WeFile
• Affordable pricing — from £27.50 per filing, with bulk discounts for accountants
• No software to install — everything runs in your browser

Whether it's your first company tax return or your hundredth, WeFile makes the process straightforward. Create your free account and file your next company tax return today.

What is a CT600 return?

A CT600 is the main form in a UK Company Tax Return. The complete filing package normally includes the CT600, relevant supplementary pages, company accounts and the Corporation Tax computation.

Who has to file a CT600?

A company or organisation must file when HMRC issues a notice to deliver a Company Tax Return. A dormant company does not automatically file every year if HMRC has not requested a return or has withdrawn the filing notice.

When is a CT600 due?

The normal filing deadline is 12 months after the accounting period ends. Corporation Tax is normally payable earlier, 9 months and 1 day after the period ends.

Can I file a CT600 myself?

Yes. Directors can self-file using suitable commercial software if they can prepare accurate accounts and tax information. Complex positions may warrant advice from a qualified accountant or tax adviser.

Is HMRC’s free CT600 filing tool still available?

No. The joint online accounts and Company Tax Return service closed on 31 March 2026. From 1 April 2026, Company Tax Returns require commercial software, while Companies House retains separate accounts-filing routes.

What software do I need to file a CT600?

Use commercial software that supports the CT600, relevant supplementary pages, tax computations and the accounts format your company needs. HMRC’s supplier list confirms stated technical capability but is not an endorsement.