Microsoft may face a European Commission investigation after Norwegian developer lodged an antitrust complaint against it.
Opera has urged that Microsoft be forced to "give consumers a genuine choice of standards-compliant Web browsers", saying that its current policy precludes other browsers.
In a statement, it accuses Microsoft of "abusing its dominant position by tying its browser, Internet Explorer, to the Windows operating system and by hindering interoperability by not following accepted Web standards".
The CEO of Opera, Jon von Tetzchner, said: "We are filing this complaint on behalf of all consumers who are tired of having a monopolist make choices for them".
Opera has asked that the Commission forces Microsoft to unbundle Internet Explorer from Windows and/or carry alternative browsers pre-installed on the desktop. And second, that Microsoft be made to follow "fundamental and open Web standards accepted by the Web-authoring communities".
It continues by accusing Microsoft of creating a standard that is "more costly to support, harder to maintain, and technologically inferior and that can even expose users to security risks".
Opera points to a ruling in September by the European Court of First Instance that ruled that Microsoft had "illegally" tied Windows Media Player to Windows.
It urges the same principles to be applied to Internet Explorer.
Opera concludes: "Opera believes that the remedies will help promote consumer rights worldwide and force Microsoft to begin competing with Opera and others on the merits of its browser".
Microsoft is not commenting on the case.