The European Commission said yesterday that there would be two further investigations into Microsoft’s commercial practices, only weeks after the American software giant had conceded defeat in a battle with regulators in Brussels.

Neelie Kroes, the Competition Commissioner, decided to act after being told that Microsoft had tied Internet Explorer, its web browser, to its Windows operating system and that it was making it difficult for competitors to work with Windows.

The first complaint was made last month by Opera Software, of Norway. The second was tabled by the European Committee for Interoperable Systems (Ecis), a nonprofit organisation that represents companies supplying information technology products.

The complaints are separate from the earlier case, in which Microsoft was fined almost €800 million (£608 million) for abusing its dominant position and was forced to unbundle its Media Player software from its Windows operating system and make interoperability information available to software developers.

However, they draw on the judgment of Europe’s second-highest court, which rejected the American company’s appeal in September. Then, the Luxembourg judges ruled that Microsoft could not exclude rivals by tying products to Windows and must allow rival software to interoperate.

A spokesman for Opera, whose software has 20 million users world-wide, said last night: “We are doing this for the future of the web. We want to give people a choice so a site can look good whatever device is used to access it.” Ecis is supporting Opera’s complaint. When it was lodged, Thomas Vinje, the organisation’s spokesman, said: “Browsers are the gateway to the internet. Microsoft seeks to control this gateway.”

The Commission made clear last night that launching the latest proceedings did not imply that it had proof of any antitrust violations but that it would investigate as “a matter of priority”. European investigators will examine whether Microsoft has refused to disclose interoperability information and will look at whether Office Open XML, its file format, is sufficiently interoperable with competitors’ products, and whether Microsoft has introduced technologies that would reduce compatibility.

Microsoft said last night that it would cooperate with the investigation. “We are committed to ensuring that Microsoft is in full compliance with European law and our obligations as established by the European Court of First Instance in its September 2007 ruling,” it said.

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