Microsoft has launched a scathing attack on the European Commission, accusing the antitrust regulator of seriously violating the software group’s right of defence and breaking its own rules of procedure.

Microsoft’s allegations – contained in a strongly-worded letter seen by the FT – will further escalate the already fierce battle between the Commission and the software giant. The group currently faces the threat of fresh financial penalties – of up to €2m a day – after the Commission formally charged Microsoft in December for failing to comply with its landmark 2004 antitrust ruling.

The Commission has threatened Microsoft with fines of up to €2m a day unless it complies.

But Microsoft now says it has not been given access to many of the documents that form the basis of these charges and that it is therefore in a poor position to counter the Commission’s claims.

In a 5-page letter was sent to the Commission on Monday, one of its lawyers writes: “The absence of access is seriously prejudicing Microsoft’s rights of defence.”

The letter alleges that the group has only been granted access to 29 out of 100 relevant documents, and that 25 of them were part of the correspondence between the Commission and Microsoft itself. Ian Forrester QC, a partner at White & Case and one of the senior lawyers working for the group, adds in the letter: It is difficult to deny there is an infringement of what most lawyers in Brussels would regard as normal rights of defence. Indeed, “I take the liberty of suggesting that normal rights of defence are being trumped by the supposed need to avert a ‘danger to effective competition’.”

A Commission spokesman said the issue of access to documents was still under discussion between Microsoft and the independent Commission official responsible for ensuring due process. “It is premature for Microsoft to allege we are riding roughshod over their rights of defence because no definitive conclusion has yet been reached on their requests.”

But Microsoft says the Commission’s stance contradicts the position outlined by Neelie Kroes, the EU competition commissioner, last year, in which she confirmed companies’ rights to access the regulator’s files. The group’s letter states: “The position taken by the Commission is particularly troubling because it contradicts the Commission’s stated commitment to increased transparency and due process in antitrust investigations.”

A spokesman for Microsoft said on Thursday: “We have great respect for the Commission as a institution but we are very concerned about the lack of transparency in this procedure.”