The UK's Competition Appeal Tribunal (CAT) has ruled that a £2.1bn ($2.8bn) lawsuit levied against Microsoft can proceed to trial.

Ruled yesterday (April 21), the lawsuit is a collective action court case brought by Dr. Maria Luisa Stasi that accuses Microsoft of overcharging for Microsoft services on rival cloud platforms. It is seeking compensation for around 59,000 businesses and organizations.

The case was filed in December 2024 and went to the CAT in December 2025. It focuses specifically on overcharging for the use of Windows Server on Google Cloud, Amazon Web Services, or Alibaba Cloud.

The CAT has now granted a Collective Proceedings Order on an opt-out basis, which enables businesses to pursue legal action as a group rather than submitting individual claims, and ruled that the claim "comfortably crosses the hurdle of having a real prospect of success.”

Dr Stasi said of the outcome: “Today’s ruling is an important moment for the thousands of organizations impacted by Microsoft’s conduct and in ensuring that a critical sector of the economy is innovative and open. For years, Microsoft’s practices have had [a] real financial impact on both public and private organizations. I’m now looking forward to preparing for trial and getting their money back on their behalf.”

Microsoft previously told DCD that the lawsuit was an "opportunistic attempt" to "piggy-back on baseless complaints Google has made and which we’ve all addressed or rebutted. We enable our cloud competitors to profit by offering our products to their cloud customers, and our competitors set their own prices when they do this.”

Following the latest ruling, a Microsoft spokesperson said: “We will pursue an appeal of the Tribunal’s decision because it failed to follow recent precedent from the Supreme Court on class action certifications. We also dispute the underlying allegations by the class representative, and today's decision makes no final determination on those claims."

The ruling comes just under one month after the UK's Competition and Markets Authority launched an SMS investigation to see if the company held "strategic market status" related to its software licensing practises that would enable the CMA to “impose targeted and bespoke interventions to address the concerns we have identified.”

The company is also being investigated to see if its business practises are anticompetitive in the EU, Brazil, Switzerland, the US, and Japan.