The European Commission has hit back against charges by Microsoft that it had colluded with the company's rivals in its ongoing anti-trust case. The Commission has released a confidential agreement with the company that, it says, proves that it has acted properly.

Microsoft had filed a formal complaint against the Commission, saying that the body was failing to act as a fair and independent regulator in the anti-trust case. The company said that the Commission encouraged Microsoft rivals such as Sun Microsystems and IBM to lobby an independent computer expert, who is supposed to advise the Commission on whether Microsoft is complying with the Commission's 2004 anti-trust ruling, to take an anti-Microsoft line.

On Friday, the Commission published a document that details the mandate of the expert, the so-called monitoring trustee. The Commission and Microsoft reached an agreement on the mandate last October. A Commission spokesman said that the document showed that it was perfectly normal for the trustee, British 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 that the document, which has been published on the Commission's website, "clarifies that communications [between the trustee and the Commission] 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 that 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. He added that the Commission 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 wants the Commission to release correspondence between rival companies, the trustee and the Commission's internal experts that, the company says, would provide evidence of the Commission's bias.

A Microsoft spokesman denied that the document allowed the Commission to deny the company access to the correspondence between the trustee, the Commission and the company’s rivals.

"Nothing in the decision published today authorises the Commission 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 Commission ruling found Microsoft guilty of exploiting the dominance of its Windows operating system in the desktop PC sector to gain market share for bundled products. As part of the ruling, the Commission 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 Commission 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 €2 million (US$2.4 million) 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 is arguments at a hearing in Brussels at the end of March.