A Dutch judge has ordered the makers of popular Napster-a-like file-sharing software KaZaA to make it impossible for users to continue to share copyright-protected material using the software, or pay a fine for each day the product remains unchanged.

In a case brought to the Amsterdam district court by Buma/Stemra, an organisation that represents composers, songwriters and music publishers in the Netherlands, Judge Orobio de Castro found that because KaZaA does not have a licence from Buma/Stemra it is acting unlawfully by making software available that allows users to download music files.

This could, in theory, be applied to other beleaguered music-swap sites. KaZaA has 14 days to comply with the order. If it doesn't, the company will have to pay a touch over £28,000 a day, up to a maximum of around £570,000, until it does.

KaZaA is puzzled by the verdict and it doesn't understand exactly what to do, said KaZaA's attorney Christiaan Alberdingk Thijm.

"Our software can't disappear, it is already out there," he said. "And the minute we take down our website, mirrors will appear," referring to exact copycat sites made available from other servers, by other individuals.

KaZaA is so-called peer-to-peer software that lets people search each other's hard drives for files they want and to download them. KaZaA doesn't require a central server to work, unlike Napster, which was shut down when it installed filters on its servers after a legal battle in the USA.

"At trial, we asked the judge to be clear in his verdict if he was going to side with Buma/Stemra. Now we don't know what to do. Should we go door-to-door and ask people to stop using the software? Should we put a notice on our website? Whatever we do, it will always lead to a new conflict with Buma/Stemra," Alberdingk Thijm said.

KaZaA is weighing its options and might appeal.

A spokesman for Buma/Stemra declined to comment, saying the company had not yet studied the verdict.

But users won't be affected by the verdict, said Lee Black, research director at analyst firm Webnoize. He called the ruling the fruit of "a judicial system that doesn't understand the technology it is trying to regulate".