Microsoft is accused of shutting out browser makers in favor of its own Internet Explorer on Windows RT, a tablet-centric version of the Windows 8 operating system designed for devices running ARM chips.
The EU watchdog is also investigating allegations that Microsoft does not provide rival browsers with access to complete APIs (application programming interfaces, which allow the OS and other programs to talk to each other) in Windows 8, according to Reuters.
Microsoft's Internet Explorer 10 browser, which comes with Windows 8, will run in both "Metro" and "Classic" modes, the former running from the Start screen, the latter running from the 'behind the scenes' desktop. Mozilla, the creator of the rival Firefox browser, claims that running a third-party browser in Classic mode is the only way Microsoft has allowed developers to access the APIs.
As a result, with the exception of IE10, browsers running on Windows RT can't take advantage of the best features of both classic and Metro modes, according to Mozilla.
"On ARM chips, Microsoft gives IE access special APIs absolutely necessary for building a modern browser that it won't give to other browsers so there's no way another browser can possibly compete with IE in terms of features or performance," Mozilla's Asa Dotzler wrote in a blog recently.
It's not clear whether the allegations relate solely to Windows RT or whether it also includes the desktop and notebook versions of Windows 8.
The issue has also gotten some attention in the U.S. In May, a Senate committee confirmed it would look into Mozilla's claim that Microsoft engaged in anti-competitive behavior by only allowing access to APIs in the "classic" mode.
A couple of days later, EU antitrust regulators said they were unsure if Microsoft's move to relegate third-party browsers to the desktop ran afoul of an existing agreement between the company and the European Union relating to the "browser ballot" screen -- a Windows update that allows European users to select a browser other than pre-installed Internet Explorer -- but said they would remain "vigilant".
The browser ballot screen question has drawn the attention of regulators again this week, after the EU announced it would look into claims that Microsoft failed to include a browser ballot screen on 28 million new PCs.
Microsoft admitted it had "fallen short in [its] responsibility," and offered to extend the period in which the company would be obliged to continue providing the browser ballot screen to EU-based Windows customers.
EU Competition Commissioner Joaquin Almunia warned: "If infringements are confirmed, Microsoft should expect sanctions."
If Microsoft is found to have flouted European antitrust laws, it could face a maximum fine of up to 10 percent of its global annual revenue, or around $7 billion, for each investigation.