The order by US District Judge Marilyn Hall Patel came in a closed door session, according to a RIAA spokesman. The order was not immediately available, he added.
RIAA chief executive Hilary Rosen in a statement said that the court ordered Napster not to resume operations until the company can show that it can comply with an earlier court order to ban trades of major label songs.
A Napster spokesman confirmed the court had issued a ruling and that the site would remain closed until further notice.
"While we are disappointed by this ruling, we will work with the technical expert to enable file transfers as soon as possible and we are continuing full steam ahead toward the launch of our new service later this summer," said Hank Barry, CEO of Napster.
The order comes more than a week after Napster began a self-imposed blackout as it seeks to install new audio fingerprint technology aimed at filtering unauthorized works from its service.
At an April 10 hearing, Patel called Napster's filtering efforts up to that point "disgraceful", saying that if a song could be found by people on the service, Napster ought to be able to block it.
"You find a way to filter out (those songs) for which you can search," she told the file-swapping service at that time, adding that if it couldn't block copyrighted songs, "maybe the system needs to be closed down".
At that hearing she also appointed A J "Nick" Nichols as a court mediator to handle technical issues related to proposed filtering solutions.
Patel had ordered Napster to begin blocking songs in early March, after the 9th US Circuit Court of Appeals asked her to revise an earlier injunction that gave the company room to continue its swapping service as long as it took all "reasonable" steps toward blocking copyrighted songs identified by the record companies.
Wednesday's decision may help shed light on a nearly 2-week-old blackout that Napster executives imposed on the company. Napster has repeatedly cited database "upgrades" as the source of its blackout.
Even before the most recent blackout, Napster wasn't functioning normally. In late June, Napster disabled old versions of its software and forced members to a new version that rendered the service unusable. The new software blocked even the most obscure, uncopyrighted works from being traded.
If Napster interprets the ruling as a complete blackout on all trading, the company's ability to test new business strategies and comply with copyright law could be seriously undermined.
The company unveiled a technique two weeks ago that allows it to identify songs by their audio "fingerprint"--literally matching the sound of musical tracks to a list of copyrighted tunes banned from the service. The technique avoids the pitfalls of filters that block songs based on file names, which can be easily changed. But it carries its own uncertainties, including significant logistical barriers in building a database of banned songs.
If Napster is allowed to continue its fingerprinting experiments, it's unlikely to help boost the number of recordings available to online consumers.
Napster claims fingerprint filtering will reverse the decline of music being traded. But it has had the opposite effect in its initial use.
Almost all the music that remained on the service vanished. According to Webnoize, the average number of files shared by people online dropped to just one. But a few days later, the company pulled the plug altogether, saying some copyrighted songs were still getting through and an "upgrade" to the database was necessary to make the new filtering technology work perfectly.
Between 100,000 and 150,000 people have remained logged into the service throughout the outage--a far cry from the 18.7 million people who were using Napster in October. According to market-research company PC Data, nearly one-fifth of the total online population downloaded free music from Napster.
The voluntary blackout, consumer defection and technical delays highlight the extraordinary logistical hurdles Napster and any other company must go through if it implements audio fingerprinting, which has been touted as one of the most surefire ways of blocking unauthorized trades of songs.
In theory, the fingerprinting technology takes a snapshot of the actual audio characteristics of a given song and sends this to Napster's central servers. This is compared against a master list of fingerprints and either given the go-ahead or blocked.
But this master list does not exist and has never existed. A company called Loudeye Technologies has rights to much of the music created by major and other North American music labels and is creating "fingerprints" from these files for Napster. These files must then be independently matched to the lists of song titles and artists that have been identified by the record companies.
Napster itself has said that the file-identification technology works and that just a few details are holding up the decision to restart the service.