Ailing online music rebel Napster has moved onto the critical list. US judge Marilyn Patel told the file-swap service on Wednesday that it must stay closed until it can prove it is not violating music industry copyrights.

The service has been down for two weeks as Napster's technical team works to integrate its new filtering system with its database. The judge has now ordered the service not to resume operations until the new filters are 100 per cent effective, which may prove very difficult to comply with.

Napster claimed at the hearing that its new filtering technology worked with 99 per cent accuracy, but record label lawyers presented evidence alleging that half of their copyrighted songs would be available for unauthorised download if Napster started up again.

Napster's chief foe, the Recording Industry Association of America (RIAA), took the opportunity to warn other file-trading websites that their days are numbered.

Hilary Rosen, president of the RIAA, said in a statement: "Judge Patel's decision today that Napster should not resume operations until it can show that it can comply was inevitable given its failure to comply with the court's order for so long.

"While we appreciate that Napster is attempting to migrate to a legitimate business model, its inability to prevent copyright infringement from occurring on its system has only hampered the development of the marketplace in which it now hopes to compete.

"In order for that marketplace to flourish, intellectual property must be defended rigorously. Today's ruling sends a clear signal to all infringers: any attempt to hide illegal activity behind the shield of technological innovation will not be tolerated."

Napster's lawyers have so far not commented. Its users aren't waiting around for it to launch a paid-for subscription service later in the year: song-trading traffic on the service was down 95 per cent on its February peak when the facility was suspended on 1 July.