The EC ruled in 2004 that Microsoft should document some code with rivals, which include IBM, Novell and Sun Microsystems, to ensure their software works as well with Microsoft’s Windows as Microsoft’s own software.

Microsoft did provide technical documentation to its rivals, but the EC deemed it inadequate and threatened Microsoft with potential daily fines of as much as $2.4m backdated to mid-December.

Microsoft argued recently that to provide additional technical information to them would deny Microsoft’s intellectual property rights.

The company is defending itself during a weeklong hearing in Europe’s Court of First Instance against the EC’s original ruling, which included $613bn in fines for also allegedly hampering competition by bundling its media software into Windows.

John Cooke, the lead judge in a 13-strong panel, has challenged the EC’s lawyers on their interoperability decision.

The information which forms interoperability is hugely valuable commercial information, said Cooke, according to Reuters.

Cooke asked the EC whether competition rules require that (to) be taken away from Microsoft, conveying a huge commercial advantage, Reuters reported.