Microsoft has written to the EC (European Commission) to complain that the company is being denied the normal rights to defence in its antitrust case. In a letter sent to the EC on Monday, a Microsoft lawyer said the EC was "seriously prejudicing Microsoft's rights of defence" in the ongoing antitrust case by denying the company access to certain files relating to the case.

The company argues that unless it gets access to these documents it will not to be able to meet the EC's 15 February deadline to prove it is complying with its 2004 antitrust ruling. The EC has threatened to fine the company €2m (about £1.37m) a day until Microsoft complies.

But the five-page letter, written by Ian Forrester QC, one of Microsoft's top lawyers, says the earliest the company could draw up its response is 28 February. Commenting on the EC's decision to extend the deadline for compliance only until 15 February, it says the decision seems to be based on the "fallacious notion" that extending the delay further would "cause a danger to effective competition".

Microsoft's rivals in the case say the longer the delay in forcing the company to comply with the ruling, the greater the risk that it completely consolidates its dominant position in the market.

The complaints centre on a number of files – correspondence between the EC, its external group of technical experts and the independent monitoring trustee who is assessing whether Microsoft is complying with the ruling.

The EC argues that it has no obligation to release the documents Microsoft is asking for because it has granted access to the final reports by the technical experts and the monitoring trustee.

But Microsoft says access to the files is essential to ensure it knows all the arguments being used by the EC to say that the company has not yet complied.

The EC has responded to the letter, saying the issue of Microsoft's access to documents "is still the subject of continuing correspondence between the hearing officer and Microsoft", according to Jonathan Todd, spokesman for European Union competition commissioner Neelie Kroes. It was therefore "premature" for the company to claim that the EC had prejudiced its right of defence.

A spokesman for Microsoft said: "All we are asking for is access to our file. This is a basic question of fairness and transparency."