EU regulators issued a fresh warning to Microsoft that the US software giant was still failing to comply with their 2004 antitrust ruling.

Seeking to turn up the heat, the European Commission sent a letter to Microsoft saying that independent observers monitoring the case agreed that the US company was not doing enough to comply with the ruling.

Microsoft is embroiled in a standoff with the commission on whether or not the group is complying with a March 2004 anti-trust decision against it, in which the company was fined a record 497 million euros (590 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 to divulge information on its operating system needed by manufacturers of rival products.

A monitoring trustee approved by Microsoft advised EU regulators that the information was "incomplete, inaccurate and unusable", the commission said, supporting its position.

Similarly, a firm advising the commission on technical aspects of the case said that the documentation provided by Microsoft was written "primarily to maximize volume (page count) while minimizing useful information", according to the commission

Brussels is increasingly impatient for evidence of compliance with its 2004 ruling and turned up the heat in December by threatening to slap a daily fine of up to two million euros on the company.

The total fine, which would be back-dated to December 15 if levied, could amount to more than 100 million euros.

The US giant is hoping to have the ruling annulled in an appeal due to go before the European Court of First Instance, the EU's second-highest court, in the last week of April.