The Competition Appeal Tribunal has certified a £1.7 billion class action against Microsoft, brought on behalf of thousands of UK businesses and public sector organisations alleged to have been overcharged for Windows Server licences when using rival cloud computing platforms.
The judgment, handed down on 21 April 2026, grants a Collective Proceedings Order (CPO) in the case of Stasi v Microsoft Corporation & Ors, brought by proposed class representative Dr Maria Luisa Stasi against Microsoft Corporation, Microsoft Limited, and Microsoft Ireland Operations Limited.
The case centres on allegations that Microsoft charges higher wholesale prices for Windows Server to users of rival cloud platforms - including Amazon Web Services, Google Cloud, and Alibaba Cloud - than it does to users of its own Azure platform, with those costs passed on to customers, making Azure artificially cheaper than its competitors.
Competition lawyer Maria Luisa Stasi is bringing the case on behalf of nearly 60,000 businesses that run Windows Server on rival cloud platforms. The collective proceedings allege that Microsoft has abused its dominant position contrary to the Chapter II prohibition in section 18 of the Competition Act 1998 and/or Article 102 of the Treaty on the Functioning of the European Union.
The tribunal said in its judgment that the claim "comfortably passes" the reasonable prospect of success threshold.
A central issue in the certification hearing was whether the proceedings should be brought on an opt-in or opt-out basis. The tribunal rejected Microsoft's argument that the claims should be bifurcated into separate opt-in and opt-out classes, and certified the claim entirely on an opt-out basis.
The tribunal applied the recent guidance of the Supreme Court in Evans v Barclays Bank Plc & Ors [2025] UKSC 48 in reaching that conclusion. The CPO is expected to be published within weeks and will set a deadline for businesses to opt out, and may include directions for when hearings begin.
Dr Stasi described the ruling as "an important moment for the thousands of organisations impacted by Microsoft's conduct", adding that Microsoft's practices had had real financial impact on both public and private organisations over many years.
James Hain-Cole, partner at Scott+Scott UK LLP, which is leading the case for the claimant, said: "Certification of this claim is a pivotal step in securing compensation for thousands of businesses and organisations. The decision illustrates the importance of the regime for UK businesses who, like consumers, require and deserve the access to justice that it was designed to offer."
A Microsoft spokesperson said the company would pursue an appeal of the tribunal's decision, stating that it "failed to follow recent precedent from the Supreme Court on class action certifications", and that Microsoft disputed the underlying allegations, noting that the certification decision made no final determination on those claims.
The case is of direct practical interest to public authorities, who are among the largest users of Microsoft products, with many running Windows Server workloads on cloud infrastructure. Those operating Windows Server on AWS, Google Cloud or Alibaba Cloud during the relevant period may fall within the class and would be included in any eventual damages award on an opt-out basis unless they take active steps to exclude themselves before the deadline set in the CPO.
The CPO will set out the class definition and the opt-out deadline in detail. Depending on the scope of the class as certified, councils could stand to receive compensation without the need to take any active steps to join the claim.
Sarah Ford KC of Brick Court Chambers, instructed by Scott+Scott UK LLP, appeared for the class representative.
