What Is the UK's Global Minimum Tax and Who Is Affected?

The Global Minimum Tax — known internationally as Pillar Two of the OECD/G20 Inclusive Framework — is a landmark international tax reform designed to prevent large multinational companies from shifting profits to low-tax jurisdictions. The UK was one of the first countries to implement Pillar Two into domestic law, with the rules applying to accounting periods beginning on or after 31 December 2023.

What Is Pillar Two?

Pillar Two introduces a global minimum effective tax rate of 15% on the profits of large multinational enterprises (MNEs) in every jurisdiction where they operate. If a company pays less than 15% effective tax in any jurisdiction, a top-up tax is collected to bring the effective rate up to 15%. The aim is to eliminate the race to the bottom on corporate tax rates that had allowed some companies to dramatically reduce their global tax bills by booking profits in low-tax territories like Ireland, Luxembourg, or the Cayman Islands.

Who Is Affected in the UK?

Pillar Two applies to MNE groups (and large-scale domestic groups) with annual consolidated revenues of at least €750 million in at least 2 of the previous 4 years. This means the vast majority of UK businesses — including all SMEs and mid-market companies — are completely unaffected. Only the largest approximately 200–250 UK-headquartered groups, and UK subsidiaries of non-UK groups meeting the threshold, need to comply.

The UK Mechanisms

The UK has implemented three key Pillar Two rules: the Qualified Domestic Minimum Top-up Tax (QDMTT), which applies to UK profits not subject to at least 15% effective tax rate; the Income Inclusion Rule (IIR), under which a UK parent company must pay top-up tax on low-taxed profits in overseas subsidiaries; and the Undertaxed Profits Rule (UTPR), which applies when other rules have not been applied by the relevant jurisdictions. The QDMTT was specifically designed to ensure HMRC collects the top-up tax rather than allowing foreign tax authorities to collect it first.

The Effective Tax Rate Calculation

The 15% effective tax rate is not simply the headline CT rate. It is calculated using a special formula based on financial accounting profits (not UK tax-adjusted profits) and covered taxes (including deferred tax). Many adjustments and safe harbours apply — including the Substance Based Income Exclusion, which carves out a return on employees and assets from the top-up calculation, reducing the top-up tax in jurisdictions with genuine substance.

UK Compliance

Affected groups must file a Pillar Two Information Return within 15 months of the accounting period end (18 months for the first period). The UK top-up tax is reported on a separate return from the standard CT600. HMRC has published extensive guidance and compliance resources at gov.uk/government/collections/pillar-2.

Small Business Takeaway

If your business has revenues below €750 million, Pillar Two has no direct impact on your tax position. However, if you are a supplier or advisor to large multinationals, understanding the framework helps you anticipate how those clients' tax strategies may change.