Microsoft accused European authorities of "secret collaboration" with the company's rivals in the antitrust probe of the US software giant.
In a complaint filed in Brussels as a supplemental response to the EU finding that the company abused its position of market dominance, Microsoft fired back with a series of charges of irregular conduct by EU authorities.
Microsoft said documents released last month "reveal that the (European) Commission has been conducting its investigation of Microsoft's compliance in secret collaboration with Microsoft adversaries and in violation of its own rules for communication with the Trustee" supervising the case.
The revelation "has disturbing implications for the manner in which the Commission has conducted this case," Microsoft said.
The dispute relates to a March 2004 decision by the European Commission -- the EU's competition watchdog -- to fine the software group a record 497 million euros (588 million dollars) for abuse of its market position.
Microsoft was also ordered to sell a version of its Windows operating system unbundled from its Media Player software and divulge information on its operating system needed by manufacturers of rival products.
But Microsoft said the contacts with its rivals were "inappropriate" and called into doubt the impartiality of the investigation.
"By encouraging and facilitating these communications to occur in the dark, and without any record of the content of the communications apparently being kept, the Commission has prevented Microsoft from knowing whether the content of the communications was distorted or accurate," the complaint said.
The US giant is hoping to get the ruling annulled in an appeal due to go before the European Court of First Instance, the EU's second-highest court, in April.
The commission is increasingly impatient for evidence of compliance with the ruling and turned up the heat in December by threatening to slap a daily fine of up to two million euros (2.37 million dollars) on the company.
But Microsoft said correspondence released February 13 at the company's request "calls into question whether the (EU) reports ... are really independent, impartial assessments .... or instead are argumentative tracts developed for the Commission with the assistance of Microsoft's competitors."
The company added that "the picture that emerges from a review of the correspondence ... is that the Commission, the Trustee, and Microsoft's adversaries were actively collaborating throughout the Fall of 2005 in a manner inconsistent with the Commission's role as neutral regulator and the Trustee's role as independent monitor."