Web browser developer Opera Software ASA asked European regulators Thursday to force Microsoft Corp. to give users a choice of internet software with its Windows operating system.

In a complaint with the European Commission, Opera also alleged that Microsoft was holding back developers from making programs that work with each other "by not following accepted web standards."

Opera, a small company in Oslo, has failed to gain a foothold in the browser market despite years of innovations. Its market share dipped even more as Mozilla's Firefox emerged to rival Microsoft's market-dominant Internet Explorer browser over the past few years. These days, Opera is stronger on making browsers for mobile devices, an area where Microsoft is not a major player.

Microsoft said Internet Explorer has been a part of Windows for more than a decade and supports a wide range of web standards.

"We will, of course, co-operate with any inquiries into these issues, but we believe the inclusion of the browser into the operating system benefits consumers, and that consumers and PC manufacturers already are free to choose any browsers they wish," Microsoft said.

A complaint can — but does not always — trigger an antitrust investigation by EU regulators.

Some of the claims echo complaints from years ago, when Microsoft was accused of using its monopoly position to wipe out Netscape by bundling its free Explorer browser with Windows.

Netscape quickly lost its dominance, and relatively few use it today. Remnants of the technology evolved into Firefox, which now has roughly 10 per cent of the market, with most of the rest going to Internet Explorer.

Opera said it was asking EU regulators to apply the principles of their landmark antitrust ruling ordering Microsoft to market a version of Windows without its media player program, even though there were few takers when it later went on sale.

EU antitrust chief Neelie Kroes had warned Microsoft that the order set a precedent for its future behaviour in other areas — such as its Office software and its new Vista operating system.

"Microsoft should bear this in mind," Kroes said. "The shop is still open, I can assure you.... There are a couple of other cases still on our desk."

EU officials last year cited possible problems with Vista's integrated security software, its internet search and copyright-protection tools, and software that would create PDF-like documents.

Opera is claiming that Microsoft abused its monopoly power as the supplier of software to most of the world's personal computers by giving away its Internet Explorer with Windows and not offering alternative programs.

It asked the commission to require that Microsoft unbundle Internet Explorer from Windows, include rival browsers in the default Windows installation, or do both.

Opera also said the European Commission should require Microsoft to adhere to its own public pronouncements to support open web standards. Over the years, Microsoft has added its own browser features that aren't part of the standards, and many websites follow the company's lead, given its market share.

"Microsoft's unilateral control over standards in some markets creates a de facto standard that is more costly to support, harder to maintain and technologically inferior, and that can even expose users to security risks," Opera said.

A group of Microsoft rivals, the European Committee for Interoperable Systems, endorsed the complaint. "Browsers are the gateway to the internet. Microsoft seeks to control this gateway," its legal counsel, Thomas Vinje, said.