• What is a CT600 return?

A CT600 is the official UK Corporation Tax return form. It tells HMRC how much profit a company made, and how much Corporation Tax it owes.

• Who needs to file one?
  • UK-registered companies must file a CT600 every year, even if they made no profit.
  • Overseas companies (including Isle of Man companies) must file one if they have UK taxable activities.

For an Isle of Man company, you may need to file a CT600 if:

• It has a UK branch or permanent establishment (e.g. an office, workshop, or store in the UK).
• It earns UK income subject to Corporation Tax (e.g. UK property rental income).
• It trades in the UK through agents or staff located here.
• It carries out activities in the UK that HMRC deems as “trading” from within the UK.

• Main Corporation Tax rates (from April 2023 onwards)

  • 19% – Small profits rate applies if profits are £50,000 or less.
  • 25% – Main rate applies if profits are £250,000 or more.
  • Marginal relief on the main rate – applies if profits are between £50,001 and £249,999.

• Deadlines

  • Filing deadline: 12 months after the end of your accounting period.
  • Payment deadline: 9 months and 1 day after the end of your accounting period.
    (Large companies may need to pay in quarterly instalments.)

• Late filing penalties

  • 1 day late: £100 penalty.
  • 3 months late: Another £100.
  • 6 months late: HMRC estimates the bill and adds a penalty of 10% of unpaid tax.
  • 12 months late: Another 10% of unpaid tax.
    Penalties are higher if you repeatedly file late.

• Late payment interest

  • Late payment interest is set at base rate plus 4% from 6 April 2025 (was plus 2.5% on or before 5 April 2025). Currently 8% in total from 27 August 2025.

How Acclaim Helps

At Acclaim, we provide multi-jurisdictional corporate services, tailored structures, and end-to-end administration from our base in the Isle of Man.

Our team delivers bespoke solutions for companies, trusts, foundations, and beyond, helping clients manage, grow, and protect their business with confidence.