UPDATE: Microsoft has responded to Tim Sweeney's Guardian piece, insisting its Universal Windows Platform is "a fully open ecosystem".
Kevin Gallo, corporate vice president of Windows at Microsoft, issued the Guardian the following statement:
The Universal Windows Platform is a fully open ecosystem, available to every developer, that can be supported by any store. We continue to make improvements for developers; for example, in the Windows 10 November Update, we enabled people to easily side-load apps by default, with no UX required.
ORIGINAL STORY: Tim Sweeney, boss of Gears of War maker Epic Games, has launched a scathing attack on Microsoft, which he accuses of trying to "monopolise game development on PC".
Writing in The Guardian, Sweeney called on the industry to fight Microsoft and its Universal Windows Platform (UWP) initiative, which he claims subverts the rights of developers and curtails users' freedom.
"In my view, this is the most aggressive move Microsoft has ever made," Sweeney said.
Microsoft has launched features exclusive to UWP, a move Sweeney views as "effectively telling developers you can use these Windows features only if you submit to the control of our locked-down UWP ecosystem".
"They're curtailing users' freedom to install full-featured PC software, and subverting the rights of developers and publishers to maintain a direct relationship with their customers."
Xbox boss Phil Spencer discussed UWP earlier this month, saying developers would benefit from being able to create apps that work across devices, such as Xbox One and Windows 10.
Sweeney, though, takes aim at Microsoft's Windows Store, which sells Universal Windows Applications.
He accused Microsoft of "structuring its operating system to advantage its own store while unfairly disadvantaging competing app stores, as well as developers and publishers who distribute games directly to their customers".
By default it is impossible to download UWP apps from the websites of publishers and developers, Sweeney complained. You can't, by default, install them, update them, and conduct commerce in them outside of the Windows Store.
"It's true that if you dig far enough into Microsoft's settings-burying UI, you can find a way to install these apps by enabling side-loading," Sweeney said. "But in turning this off by default, Microsoft is unfairly disadvantaging the competition. Bigger-picture, this is a feature Microsoft can revoke at any time using Windows 10's forced-update process."
Sweeney called on Microsoft to make UWP an open platform, and he pulled no punches in expressing concern for the future.
"Valve's Steam distribution service is booming with over 100m users, and publishers like Adobe, Autodesk, Blizzard, Riot Games and EA are operating highly successful businesses selling their games and content directly to consumers.
"Microsoft's situation, however, is an embarrassment. Seven months after the launch of Windows Store alongside Windows 10, the place remains devoid of the top third-party games and signature applications that define the PC experience. Where's Photoshop? Grand Theft Auto V? Fifa 2016? There are some PC ports of what were great mobile games, and some weirder things, such as the Windows 10 port of the Android port of the PC version of Grand Theft Auto from 2004.
"But the good PC stuff isn't there, with the exception of Microsoft's own software products. Does Microsoft really think that independent PC developers and publishers, who cherish their freedom and their direct customer relationships, are going to sign up for this current UWP fiasco?
"In my view, if Microsoft does not commit to opening PC UWP up in the manner described here, then PC UWP can, should, must and will, die as a result of industry backlash. Gamers, developers, publishers simply cannot trust the PC UWP 'platform' so long as Microsoft gives evasive, ambiguous and sneaky answers to questions about UWP's future, as if it's a PR issue. This isn't a PR issue, it's an existential issue for Microsoft, a first-class determinant of Microsoft's future role in the world."
Sweeney said Epic had spoken with Spencer and Microsoft to express its concern about UWP ahead of its announcement. But, it seems, this concern had fallen on deaf ears.
"Because they listened very patiently, I hoped and believed that Microsoft would do the right thing, but here we are. Microsoft's consumer launch and PR around UWP are in full swing, and this side of the story must be told."
Sweeney's disgust at Microsoft over UWP signals a remarkable souring of a relationship that once saw Epic influence the design of the Xbox 360 and create one of its defining exclusive franchises: Gears of War.
But in recent years Epic ditched the Gears of War franchise, selling it off to Microsoft, and has set upon building new PC-focused games, such as Paragon (a PlayStation 4 console exclusive), a new Unreal Tournament and Fortnite. Microsoft, meanwhile, is building Gears of War 4 - surely a UWP app - in-house.
We've asked Microsoft for a response.