Facing accusations by Microsoft that it has colluded with the company's rivals in its ongoing antitrust case, the EC (European Commission) has hit back. Yesterday the EC released a confidential agreement with the company that, it says, proves that it has acted properly.

Last Thursday, Microsoft filed a formal complaint against the EC, saying that the body was failing to act as a fair and independent regulator in the antitrust case. The company said the EC encouraged Microsoft rivals such as Sun and IBM to lobby an independent computer expert, who is supposed to advise the ECon whether Microsoft is complying with its 2004 antitrust ruling, to take an anti-Microsoft line.

Yesterday the EC published a document that details the mandate of the expert, the so-called monitoring trustee. The EC and Microsoft reached an agreement on the mandate last October. An EC spokesman said the document showed that it was perfectly normal for the trustee, UK computer scientist Professor Neil Barrett, to have contacts with rival firms.

"It is part of the obligation of the trustee to proactively make contact with third parties and discuss what information they require to make interoperable products," said Jonathan Todd, spokesman for Competition Commissioner Neelie Kroes.

In addition, Todd said the document, which has been published on the EC's website, "clarifies that communications [between the trustee and the EC] are classified as internal documents" and as such are not accessible to Microsoft.

Todd added that the trustee was a "leading and very well-regarded computer expert who was recommended by Microsoft".

Todd said the decision to publish the document detailing the trustee's mandate had been taken following consultations with Microsoft that could have prevented its release but did not do so.

Todd said the EC would not normally publish this type of document but had decided to do so in response to allegations that the trustee had been acting in an "inappropriate manner".

Microsoft is calling for the release of correspondence between rival companies, the trustee and the EC's internal experts that, the company says, would provide evidence of bias.

A Microsoft spokesman denied that the document allowed the EC to deny the company access to the correspondence.

"Nothing in the decision published today authorises it to sidestep the file access and other due process guarantees to which all companies are entitled under European law and which have been neglected in this case," said Microsoft spokesman Tom Brookes.

A 2004 EC ruling found Microsoft guilty of exploiting the dominance of its Windows OS (operating system) in the desktop PC sector to gain market share for bundled products. As part of the ruling, the EC ordered Microsoft to release a version of its OS without Windows Media Player and provide documentation that would allow rival products to interoperate with its workgroup server software.

The EC argues that Microsoft has not yet provided the necessary documentation to prove it is complying with the ruling and is threatening to fine the company €2m (about £1.37m) a day unless it complies.

Microsoft says it has done everything it can to comply, including offering access to the relevant source code in Windows. It will present its arguments at a hearing in Brussels at the end of March.