Competition Appeal Tribunal certifies £1.7 billion opt out collective proceedings against Microsoft

21/04/26

In its judgment handed down on 21 April 2026 the Competition Appeal Tribunal granted an application for a collective proceedings order made by Proposed Class Representative Dr Maria Luisa Stasi against Proposed Defendants Microsoft Corporation, Microsoft Limited and Microsoft Ireland Operations Limited (“Microsoft”).

The collective proceedings combine standalone claims alleging that Microsoft has abused its dominant position contrary to the Chapter II prohibition in s. 18 Competition Act 1998 and/or Article 102 of the Treaty on the Functioning of the European Union by reason of its licensing practices in respect of its operating system Windows Server.  

The collective proceedings are brought on behalf of a Class of UK businesses and organisations who it is contended were overcharged for licensing Windows Server on Microsoft’s rivals’ cloud computing platforms. The Class Representative claims aggregate damages estimated to be in excess of £1.7billion.

The judgment will be of particular interest on the issue of opt-in versus opt out collective proceedings since the Tribunal applied the recent guidance of the Supreme Court in Evans v Barclays Bank Plc & Ors [2025] UKSC 48. The Tribunal rejected Microsoft’s contention that the claims should be bifurcated into an opt-in and an opt-out class and certified the claim on an opt-out basis.