A US judge has said that the law governing the trading of music files over the internet is unclear.

District Judge John D Bates was hearing a test case brought by the Recording Industry Association of America (RIAA) which aims to help put an end to the online swapping of copyright music.

The music companies want to force internet service provider Verizon to reveal the identity of a subscriber who allegedly used its services to trade copyrighted songs.

We don't want to be the policeman in this process

Eric Holder, Verizon

If they can obtain the names and addresses of song-swappers - without going to court first - they can bombard them with written warnings ordering them to stop.

The US music industry says online song-swapping is depriving it of tens of millions of dollars every year.

But Judge Bates said there were ambiguities in the Digital Millennium Copyright Act.

The 1998 act was intended to uphold copyright laws on the internet but protect internet service providers from legal liability in piracy cases if they co-operate with enforcement efforts.

Congress "could have made this statute clearer," he said.

"This statute is not organised as being consistent with the argument for either side."

The judge said he would try to rule quickly, but lawyers on both sides could not estimate when a decision might arrive.

Privacy plea

Verizon argued that it should not be required to monitor its users' activities.

It said that a ruling in favour of the music industry would breach its subscribers' privacy, and set a precedent which could force other internet companies to provide their customers' names.

"We don't want to be the policeman in this process," Verizon's lawyer, Eric Holder, said.

But the judge replied: "You're not really being a policeman."

The subscriber in the test case has amassed a collection of more than 600 songs which can be downloaded by other internet users with the right software.

Sites shut down

Analysts say the judge's ruling could determine how much help record labels can expect from internet service providers as they try to stop online song-swapping.

Since the 1998 law, internet service providers have complied with nearly 100 requests to close down sites that contain copyrighted material.

But new peer-to-peer systems like Napster have meant that song-swappers can bypass websites and download music stored on the hard drives of other personal computers.

The music industry is now going after the individual peer-to-peer users.

Verizon says it would be unfair to cancel users accounts unless the music companies concerned filed formal legal proceedings that would give the users a chance to fight back.

But the music industry says that would take too long.

Letters 'enough'

Companies believe that most song-swappers would stop accessing copyrighted material if directly approached.

"One of the things we're discovering is that people are not aware that that they are engaging in conduct which is clearly illegal," said RIAA lawyer Cary Sherman.

"If you got a letter from the RIAA saying we know that you're doing this, I'd say there's a good chance that you would stop."

Napster, which pioneered the software which made song-swapping possible, was temporarily shut down last year following legal action by record companies.

But it has been succeeded by a new wave of online song-swapping services, including Kazaa, Morpheus, and Gnutella.

Many of these new services have added technical innovations which make them less vulnerable to lawsuits from established music companies.