Napster must block access to copyrighted content through its directory of songs archived by the service’s users, the 9th U.S. Circuit Court of Appeals said. Napster officials have said such a ruling could force them to shut down the service.

In a 58-page opinion, a three-judge panel told a lower court judge to rewrite her injunction to focus more narrowly on the copyrighted material. The panel also directed the Redwood City-based company to remove links to users trading copyrighted songs stored as MP3 files.

“This is a clear victory. The court of appeals found that the injunction is not only warranted, but required. And it ruled in our favor on every legal issue presented,” said Hilary Rosen, president and chief executive of the Recording Industry Association of America.

“We are disappointed in today’s ruling,” said Napster CEO Hank Barry. Under this decision Napster could be shut down – even before a trial on the merits. …While we respect the Court’s decision, we believe, contrary to the Court’s ruling today, that Napster users are not copyright infringers and we will pursue every legal avenue to keep Napster operating.”

Napster can stay in business until U.S. District Judge Marilyn Hall Patel retools her injunction, which the appellate court called overly broad.

Millions of users had flooded the company’s computer servers this past weekend to download free music, fearing an immediate shutdown of the service that has changed the face of music. Napster has an estimated 50 million users.

Webnoize, which monitors the digital entertainment economy, estimated that 250 million songs were downloaded using Napster over the weekend. Webnoize said that, on average, 1.5 million users were logged on to the system at any one time.

Major record labels hope Monday’s ruling will force millions of computer users to start paying for music the online music swapping service has allowed them to get for free.

The digital music technology Napster made popular is here to stay either way. The recording industry appears stymied by the notion of funneling music to consumers via the Internet for a price while freely available computer applications allow even the computer novice to do it for free.

The five largest record labels – Sony, Warner, BMG, EMI and Universal – sued as soon as Napster took off, saying it could rob them of billions of dollars in profits.

In May 1999, Napster founder Shawn Fanning released software that made it easy for personal computer users to locate and trade songs they had stored as computer files in the MP3 format, which crunches digital recordings down to manageable lengths without sacrificing quality.

The concept of “peer-to-peer” song trading quickly proved too popular to contain. As Napster users grew by the millions, other file-sharing programs also popped up, such as Gnutella and Freenet. And the labels themselves are looking to use the same technology, only with paying subscribers and secure digital formats that prevent copying.

After the appellate judges began deliberating in October, Napster made agreements with former business foes like Bertelsmann AG, the parent company of the BMG music unit. The German media giant has promised much-needed capital if Napster switches to a subscription-based service that pays artists’ royalties.

The other four major labels have not reached any similar agreement, apparently willing to take their chances on a final victory over Napster in the courts. Barry says Napster is ready for them.

“The largest and most successful media companies in the world have taken aim at our more than 50 million users, and today they have landed a blow. We will respond and deal with this situation in the courts,” Barry said.