The legal battle between the recording industry and free music-swapping service Napster heated up on Friday, when the Recording Industry Association of America (RIAA) urged a federal appeals court to uphold a ruling that would effectively shut down Napster.

In a flurry of legal activity surrounding the case, the U.S. government, the Motion Picture Association of America and the Microsoft-backed Business Software Alliance also chimed in on the case on Friday, by each filing so called friend-of-the-court briefs supporting the recording industry's case.

In its brief filed with the federal appeals court in San Francisco, the RIAA argued that U.S. District Court Judge Marilyn Hall Patel was correct when she ruled 26 July that Napster's file-swapping service contributes to copyright infringement on a massive scale and issued a preliminary injunction against the company.

In its 74-page brief Friday, the RIAA's lawyers urged the appeals court to reinstate the injunction against Napster immediately. They restated many of the opinions offered by Judge Patel in her ruling, in which she dismissed all of Napster's key defences.

Napster's service allows users to trade digital music files in violation of copyright laws, the RIAA's lawyers wrote, and Napster knowingly facilitates this illegal swapping with the intention of profiting from it, making the company also liable.

"Every day, Napster enables, encourages, and directly benefits from the infringement of 12 (million) to 30 million copyrighted works," the RIAA said in its brief.

The brief, which had been expected, was co-submitted with the National Music Publisher's Association and is intended to sway the opinion of the appeals court when it convenes 2 October to decide Napster's fate.

The RIAA filed its lawsuit against Napster in December last year on behalf of the major record labels it represents. In a statement on Friday, Hilary Rosen, president and chief executive officer of the RIAA, professed confidence that the appeals court will find in the recording industry's favour.

"We are not suing a technology," Rosen said. "We are suing a company that is stealing work that does not belong to them."

Napster's attorneys argued that Judge Patel misinterpreted copyright laws in reaching her decision and underestimated the impact that so-called "peer-to-peer" file-sharing services will have on the Internet.

The company also argued that its service is protected by the Home Recording Act of 1992, which allows users to make recordings of music for personal use but not for commercial distribution.

If Napster's users are not violating the Home Recording Act, then Napster itself is also protected because it merely provides a service to facilitate the recording, the company has argued.

That defence appeared to suffer a blow on Friday when the U.S. Copyright Office chimed in on the case, in a brief co-submitted with the civil division of the Justice Department and the U.S. Patent and Trademark Office.

The government's brief argued that Napster's service is not protected by the Home Recording Act, in part because the Act does not apply to music that is copied using PCs and hard drives.