Microsoft is fighting off pressure to license its Windows operating system source code to other companies in the ongoing European anticompetition lawsuit against the software maker, according to a source familiar with the case.

The source said it appears Microsoft is focusing its defence in the European case on the fact that Europe has signed up to the international intellectual-property agreement known as TRIPS, or Trade-Related Aspects of Intellectual Property Rights.

"Copyright is their defence, and there is room for debate, but ultimately I think their argument is flawed," the source said.

In a written response to the European Commission's allegations that Microsoft has abused its dominant position in the operating-system market, Microsoft argued that compulsory licensing of its codes would breach international copyright law, said the source, who has an edited public version of Microsoft's confidential reply that was filed with the European regulator on 16 November.

This 102-page version was handed out to other companies involved in the lawsuit, including Sun, whose complaint to the European competition regulator in 1998 sparked the investigation into the software maker.

However, contrary to some recent reports that quote from this nonconfidential version of Microsoft's reply, Sun is not pushing for full source code disclosure, according to a person close to the company.

"[Sun] believe the anticompetitive behavior could be addressed without Microsoft revealing its source code," said the source, adding that "the code can be described so that other companies can implement it".

Microsoft faces fines of up to 10 percent of its global sales if the Commission finds it guilty of abusing its dominant position. It may also be forced to make structural changes to its business practices in order to allay the European Commission's concerns.

Commission spokeswoman Amelia Torres declined to comment on the leaking of Microsoft's written response.

Microsoft's senior counsel in Europe, John Frank, declined to comment on any details in the company's reply. "I thought this was supposed to be a confidential document," he said, referring to published reports about the document. "I don't think it helps anyone to have selected quotes leaked."