The owner of Steam - the largest digital distribution platform for PC games in the world - is being sued for £656m.
Valve Corporation is being accused of using its market dominance to overcharge 14 million people in the UK.
"Valve is rigging the market and taking advantage of UK gamers," said digital rights campaigner Vicki Shotbolt, who is bringing the case.
Valve has been contacted for comment.
The claim - which has been filed at the Competition Appeal Tribunal, in London - accuses Valve of "shutting out" competition in the PC gaming market.
It says Valve "forces" game publishers to sign up to so-called price parity obligations, preventing titles being sold at cheaper prices on rival platforms.
Ms Shotbolt says this has enabled Steam to charge an "excessive commission of up to 30%", making UK consumers pay too much for purchasing PC games and add-on content.
The case is what is known as a collective action claim, which means that one person goes to court on behalf of a much larger group of people.
Ms Shotbolt - who accuses Valve of breaching UK competition law for at least six years - says she is bringing the claim "to stop this unlawful conduct and help people get back what they are owed."
Steam is a PC gaming platform where players can buy games and purchase in-game extras.
According to VG Insights, it had record-breaking sales in 2023, generating over $9bn (£7.1bn) in global revenue.
This was driven by 580m games sold and 14,000 new games released.
However, the market is extremely concentrated, with the top 10 bestselling games making up 61% of all sales.
The top 100 games account for 91% of total sales.
The claim is backed by legal firm Milberg London LLP, which brings group action cases against large companies.
"Competition law is there to protect consumers and ensure that markets work properly," said Natasha Pearman, a partner at the law firm.
"When they don’t work properly and consumers are harmed, collective actions of this kind provide consumers with a voice and a way of holding big companies, like Valve, to account."
It is the latest in a series of collective action legal cases against big tech.
Other claims lodged at the Competition Appeal Tribunal have sought compensation from Facebook, Google and Sony.
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