The European Commission's battle with Microsoft over its browser took another turn this week after a new ally joined the legal fight against Internet Explorer.

The European Committee for Interoperable Systems (ECIS), a trade group representing rival software companies including Oracle, Sun and IBM, has joined as an interested third party, entitling it speak at a hearing, if Microsoft chooses to have one.

In January the Commission, Europe's top antitrust authority, accused Microsoft of distorting competition in the internet browser market by attaching its IE browser to the near-ubiquitous Windows operating system.

"This is an important case to ensure that browsers can compete on the merits and that consumers have a true choice in the software they use to access the World Wide Web," said Thomas Vinje, a partner in the Brussels office of law firm Clifford Chance and spokesman for ECIS.

Vinje is already directly involved in the case because he also works directly for Opera Software, the Norwegian browser developer that sparked the antitrust case by filing a complaint to the Commission.

The Commission has three other allies, in addition to Opera: the Mozilla Foundation, maker of the Firefox browser; Google, developer of the Chrome browser, and the industry group Free Software Foundation Europe.

"Despite consistently lower user satisfaction ratings for IE, the Microsoft browser maintains its dominant position as the gateway to the World Wide Web because of illegal bundling with the Windows operating system," Vinje said in a statement, adding that "smaller, more innovative browser developers need a level playing field" to have a fair chance.

"That is why there is such broad support for the Commission's preliminary findings of abuse," Vinje said.

Microsoft has until 21 April to reply to the formal statement of objections it received from the Commission in January. It has the option of just submitting a written response, or of asking in addition for a face-to-face hearing with regulators, Opera and the third parties.