How to File Corporation Tax Online (CT600 Guide)

The CT600 is the form that every UK limited company must file with HMRC to report its taxable profits and calculate the Corporation Tax due. Even if your company made a loss or had no trading activity during the accounting period, you are still required to file a CT600 return. The form covers your company's income, expenses, capital allowances, losses, and any reliefs or deductions you are claiming. Filing must be done online through HMRC's Corporation Tax online service or compatible third-party software.

Your Corporation Tax return must be filed within 12 months of the end of your company's accounting period. However, the Corporation Tax payment deadline is earlier: you must pay any tax owed within 9 months and 1 day of the end of the accounting period. For example, if your company's accounting period ends on 31 March 2026, your payment deadline is 1 January 2027 and your filing deadline is 31 March 2027. Missing these deadlines results in automatic penalties and interest charges.

Before you can file your CT600 online, you need to register your company for Corporation Tax with HMRC. This should be done within 3 months of your company starting to trade. HMRC will send you a Unique Taxpayer Reference (UTR) number by post, which typically arrives within 2 to 4 weeks. You will also need to set up a Government Gateway account and enrol for the Corporation Tax online service. This process can take up to 7 working days as HMRC sends an activation code by post.

Before completing the CT600, you must prepare your company's statutory accounts for the accounting period. These include a profit and loss account (income statement), balance sheet, and notes to the accounts. Your accounts must be prepared in accordance with UK Generally Accepted Accounting Practice (UK GAAP) or International Financial Reporting Standards (IFRS). Most small companies use FRS 102 Section 1A (the small entities regime) or FRS 105 (the micro-entities regime), which have simplified reporting requirements.

The CT600 form has several sections covering different aspects of your company's tax affairs. The main sections include: company information and accounting period dates, turnover and income, trading profits or losses, capital allowances and deductions, chargeable gains, losses brought forward, and the final tax calculation. You must also submit your company accounts and tax computations as iXBRL-tagged documents alongside the CT600. HMRC's free online filing service can generate the iXBRL tagging for you, or you can use commercial software.

• Box 145: Total turnover from trade
• Box 155: Gross profit or loss from trade
• Box 172: Net trading profits after allowable deductions
• Box 235: Total profits chargeable to Corporation Tax
• Box 440: Tax payable after reliefs and deductions
• Box 470: Tax already paid (including payments on account)
• Box 480: Tax outstanding or overpaid

Once your CT600 is complete and your iXBRL accounts are ready, you can submit everything through HMRC's online service. After submission, you will receive an acknowledgement with a unique submission reference number. Keep this for your records. Payment can be made by bank transfer (Faster Payments, BACS, or CHAPS), direct debit, or online banking. HMRC no longer accepts credit card payments for Corporation Tax. Allow adequate processing time if paying by BACS (3 working days) or direct debit (must be set up at least 5 working days before the deadline).

• Forgetting to claim capital allowances on equipment, vehicles, and machinery
• Not including directors' loan account balances on supplementary form CT600A
• Omitting Research and Development tax relief claims where applicable
• Using the wrong accounting period dates on the return
• Failing to submit iXBRL-tagged accounts alongside the CT600
• Missing the payment deadline even though the filing deadline is later