Opera Software has filed an antitrust suit against Microsoft in the European Union, accusing it of stifling competition by tying its Internet Explorer Web browser to Windows, the Norwegian company said Thursday.

The complaint, which was filed with the European Commission on Wednesday, says Microsoft is abusing its dominant position in the desktop PC market by offering only Internet Explorer as a standard part of Windows, and hindering interoperability by not following accepted standards with IE.

Opera is asking the Commission, the executive branch of the European Union, to force Microsoft to unbundle IE from Windows, or include other browsers as a standard part of its operating system. It also wants it to require Microsoft to adhere to industry standards with its Web browser.

The issue of standards is seen as important because if all Web browsers do not use the same standards, Web site developers are likely to design their Web sites to work with the most widely-used browser, which is Internet Explorer. That gives people a disincentive to use other browsers.

Microsoft's spokesman in Brussels did not immediately have a comment on the lawsuit. The company has argued in the past that consumers benefit from its tight integration of IE and Windows.

Opera said it filed the complaint on behalf of all consumers who are tired of having a monopolist make choices for them.

The IDG News Service is a Network World affiliate.