A storm is brewing between Microsoft and one of its top developer allies—and it could get ugly fast.

Tim Sweeney, co-founder of Epic Games, accuses the Xbox maker of trying to monopolize game development for PCs and calls on developers to stand up against Microsoft, in an op-ed published by The Guardian on Friday.

“In my view, this is the most aggressive move Microsoft has ever made,” says Sweeney. “They are working to turn today’s open PC ecosystem into a closed, Microsoft-controlled distribution and commerce monopoly, over time, in a series of steps of which we’re seeing the very first.”

It’s especially noteworthy that Sweeney would come out so strongly against Microsoft. Epic Games is a longtime ally of the company and the creator of the Gears of Wars series, which is one of the tentpole franchises on the Xbox. (Epic sold the rights to the franchise to Microsoft in 2014.)

At issue, Sweeney says, is the relatively new Universal Windows Platform, which allows developers to create a single program to run on all Windows devices, ranging from PCs and tablets to phones and virtual reality headsets. But to utilize those features, developers must agree to Microsoft’s closed UWP ecosystem.

“I’m not questioning the idea of a Windows Store,” Sweeney writes. “I believe Microsoft has every right to operate a PC app store, and to curate it how they choose. … The specific problem here is that Microsoft’s shiny new ‘Universal Windows Platform’ is locked down, and by default it’s impossible to download UWP apps from the websites of publishers and developers, to install them, update them, and conduct commerce in them outside of the Windows Store.”

Microsoft’s Windows Store, where players can buy PC titles directly from the company, has been a marginal player in the gaming world. Even today, it lacks many major titles. Valve Software’s Steam, which supports an open development ecosystem, is the dominant player in the industry.

Sweeney says he believes Microsoft is using its size and power to launch a long-term effort to dominate that market. In his memo, he petitions developers to reject the effort.

“Epic has prided itself on providing software directly to customers ever since I started mailing floppy disks in 1991,” he says. “We wouldn’t let Microsoft close down the PC platform overnight without a fight, and therefore we won’t sit silently by while Microsoft embarks on a series of sneaky maneuvers aimed at achieving this over a period of several years.”