Despite Microsoft's public vows, Epic Games' Tim Sweeney says he remains concerned that Microsoft's maneuvers for its Windows 10 store are a play for creating a monopoly, and ultimately bad for games.

Earlier this year, the Epic Games co-founder wrote an opinion piece for The Guardian in which he said that Microsoft is seeking to monopolize software development and distribution on Windows 10.

Sweeney wrote about the Universal Windows Platform (UWP) initiative that is part of Windows 10, calling it a "closed platform-within-a-platform" and Microsoft's "first apparent step towards locking down the consumer PC ecosystem and monopolising app distribution and commerce."

Microsoft's Phil Spencer, the head of Xbox, refuted the claims, saying that UWP is a "fully open ecosystem, available to every developer, and can be supported by any store."

Sweeney's concern centers not on Microsoft's UWP — which is designed to allow software developers to write a single application that can run on a wide variety of Windows 10 devices, from smartphones and tablets all the way up to computers and the Xbox One — but on how those programs are treated in the Windows Store.

He believes Microsoft is prioritizing UWP programs in the store in a way that could hurt non-Microsoft stores like Steam and GOG.com.

At the time, Spencer said that Microsoft was working to address his concerns, and that more would be discussed at Microsoft's Build conference at the end of March.

But Sweeney says the discussions during Build and other one-on-one talks have done nothing to change his opinion.

"We've continued to discuss Epic's concerns directly with high-level Microsoft folks," he said, "and are grateful for their willingness to engage in substantive discussions.

"However, no conclusion is yet in sight."

Last week during E3, Microsoft announced a new program meant to, in part, expand the reach and use of the Windows Store.

The Xbox Play Anywhere program will allow people to purchase certain games for their Xbox One or Windows 10 PC, and be able to play it on either machine. The initiative will also maintain game progress in a single file for both machines.

The service requires Windows 10 Anniversary Edition (coming this summer), the latest update on the Xbox One, and, of course, the use of the Windows Store.

So far, Xbox Play Anywhere supports about a dozen games, with more to come.

While Sweeney remains skeptical of the Windows Store's long-term openness, he was delighted with the news of Play Anywhere.

"I think this is great," he told Polygon. "Gamers really want the ability to buy a game once, and access it on all devices and stores.

"All game developers and ecosystem companies should work together to make the experience as seamless as possible for customers. On Paragon, we worked closely with Sony to ensure that PlayStation 4 and PC gamers can cross-buy and sync their progress. Good Old Games now syncs its library with Steam purchases."