Every UK limited company must file a corporation tax return (CT600) with HMRC. Whether you're running a one-person company or a growing SME, understanding the CT600 process is essential to staying compliant and minimising your tax bill. This guide covers everything you need to know — including the critical CATO portal closure coming in March 2026.

What Is the CT600?

The CT600 is the official corporation tax return form that UK companies file with HMRC. It reports your company's taxable profits and calculates the corporation tax due. Along with the CT600, you typically submit:

  • Your company's statutory accounts (usually in iXBRL format)
  • A tax computation showing how taxable profits were calculated from the accounting figures
  • Any supplementary pages relevant to your company's activities

The CT600 covers the company's accounting period (usually 12 months). If your accounting period is longer than 12 months, you'll need to file two returns.

Key Deadlines

Corporation tax deadlines run from the end of your accounting period:

  • 9 months + 1 day after the accounting period ends — corporation tax payment deadline
  • 12 months after the accounting period ends — CT600 filing deadline

For example, if your accounting period runs to 31 March 2026:

  • Corporation tax payment due: 1 January 2027
  • CT600 filing deadline: 31 March 2027

Late filing penalties start at £100 and rise to £200 after 3 months. After 6 months, HMRC estimates your tax and adds 10% of the unpaid amount. After 12 months, the penalty rises to 20% of the unpaid tax.

CATO Portal Closure — 31 March 2026

This is urgent: HMRC's Company Accounts and Tax Online (CATO) portal is closing on 31 March 2026.

CATO has been the free online tool that many small companies (and their accountants) use to prepare and file CT600 returns and company accounts directly with HMRC and Companies House. After 31 March 2026, this option will no longer be available.

If you currently use CATO, you need to switch to commercial software before the closure date. Don't wait until the last minute — transition now to avoid disruption to your filing schedule.

What You Need to File

To prepare your CT600 return, gather:

  • Trial balance — your year-end accounting figures
  • Bank statements — for reconciliation and verification
  • Fixed asset register — details of all capital items for capital allowance claims
  • Loan agreements — for interest deductions and director's loan account (s.455) calculations
  • Dividend records — for dividends paid during the period
  • Previous year's CT600 — for loss carry forward figures and brought forward capital allowances
  • Details of associated companies — affects marginal relief calculations and quarterly instalment thresholds

Key Calculations in the CT600

Adjusted Trading Profits

You start with the accounting profit and make adjustments: add back disallowable expenses (client entertaining, depreciation, personal expenses), deduct capital allowances, and apply any other adjustments. The result is the taxable trading profit.

Capital Allowances

For the year ended in 2025/26, key rates include:

  • Annual Investment Allowance (AIA) — 100% on the first £1,000,000 of qualifying expenditure
  • Full expensing — 100% first-year allowance on qualifying plant and machinery
  • Writing Down Allowance (main pool) — 18% reducing balance
  • Writing Down Allowance (special rate) — 6% reducing balance
  • Structures and Buildings Allowance — 3% straight-line

Corporation Tax Rates

For accounting periods starting on or after 1 April 2023:

  • Small profits rate: 19% (profits up to £50,000)
  • Main rate: 25% (profits over £250,000)
  • Marginal relief: profits between £50,000 and £250,000 (effective rate between 19% and 25%)

These thresholds are divided by the number of associated companies plus one.

How TaxStats AI's CT600 Software Works

TaxStats AI simplifies CT600 preparation for small companies and their accountants:

  • AI-guided workflow — enter your trial balance and the AI guides you through adjustments, asking the right questions about your company's activities
  • Auto-calculated adjustments — the AI identifies disallowable expenses, calculates capital allowances across all pools, and applies loss reliefs
  • Tax computation generation — generates a formatted computation in XLSX or PDF showing the route from accounting profit to tax liability
  • Associated company rules — handles marginal rate calculations including the impact of associated companies
  • R&D tax credits — assess qualifying expenditure and calculate RDEC relief
  • CATO replacement — full CT600 preparation and filing capability as a replacement for the closing CATO portal

Start free with TaxStats AI. Professional plans from £9.99/month unlock full document generation for CT600 computations. Practice plans from £29.99/month add multi-client management for accounting firms.