Apple and Google have removed Fortnite from their app stores, preventing players from installing one of the world's most popular games on to their phones.
Apple said Epic had taken the "unfortunate step of violating the App Store guidelines", but the game developer appeared to know the ban would come, announcing it had filed a legal action minutes after the removal.
Why is Fortnite taking on these tech giants?
"The criticism is essentially an anti-competition one," says James Clayton, the BBC's technology reporter.
"Apple and Google run the operating systems of pretty much all of the phones in the world. That means they get to choose who can run apps on their stores, and who can't. They also get to set the charges."
Many developers believe this 30% charge of in-app purchases is unfair, and Epic Games is hoping for new rulings that would change the way Apple and Google run their app stores.
"Epic is seeking... to allow fair competition in these two key markets that directly affect hundreds of millions of consumers and tens of thousands, if not more, of third-party app developers," it said.
Epic also said it was not looking to get any money from the legal case.
Can I still play Fortnite on my phone?
On iPhones and iPads, the App Store is the only way to properly load apps. But, Apple said Epic had updated their game "with the express intent of violating the App Store guidelines".
Epic said any iPhone players who already have the app installed should be able to continue playing until the game's next update rolls out. After that, they will lose some features.
Those on an Apple Mac computer will not be affected, since that version does not use the iOS App Store.
In a statement, Apple said the rules were applied equally to every developer but added that it would try to work with Epic to bring Fortnite back.
Google also appeared to remove the app from its Google Play Store - though it remains available on Android phones through other means, such as Epic Games' own site.