IBM, Oracle, Nokia, Real Networks and Red Hat have all lined up against Microsoft in the software giant's ongoing dispute with the European Commission.

Microsoft's appeal against the EC's anti-trust case heard on Friday that the European Committee for Interoperable Systems (ECIS) had been accepted as an intervenor in the case.

The ECIS is an umbrella group supporting the Commission's efforts to punish Microsoft for abusing its monopoly position and the five major IT companies have all applied to participate within it.

Intervenors have the opportunity to access the court's case files and provide written observations on the deliberations. The CFI announced the list of official intervenors for both sides last March, but the word was still out on ECIS since it had filed its application late.

The acceptance of ECIS makes no difference from Microsoft's point of view, according to its spokesman. The ECIS members were already participating through other intervening groups, he said.

IBM, Oracle, Nokia, RealNetworks, and Red Hat also all part of the Software and Information Industry Association (SIIA), which was already accepted as an official intervenor on the EC's side. Real Networks has intervenor status on its own, having helped kick off the anti-trust case by arguing that Microsoft was squeezing it out of the media player market by bundling its Windows Media Player with the Windows operating system.

The five vendors were prompted to file for intervenor status under ECIS after they saw Microsoft settling with long-time detractors, such as Sun and the Computer and Communications Industry Association (CCIA), representatives for the group have said. The group hoped to lend weight to the Commission's case, fearing that the settlements would be seen as waning industry support.

The Commission decided in March last year that Microsoft had abused its market dominance in the PC operating systems market, and slapped it with a 497 million euro fine. It also ordered the company to sell a version of Windows in Europe without Windows Media Player and give rivals access to information that would allow them to make their workgroup server products work better with PCs running Windows.

Microsoft appealed the rulings in June and the case is now winding its way through the European legal system.