The Dutch Supreme Court on Friday ordered Internet company Lycos to reveal the identity of a client in a benchmark decision on privacy that was praised by copyright groups as a way to go after illegal swapping of music and movies online.

It is the first ruling of its kind in the Netherlands on Internet privacy and could have far reaching consequences for other Internet providers.

The country's highest court ruled that Lycos had wrongly protected the identity of a user who anonymously posted slanderous allegations against an Internet postage stamp dealer on a member site. The dealer, who traded stamps on auction site e-Bay, was accused of cheating buyers.

The claimant, identified in court documents only as A. Pessers, took Lycos to court in 2003, seeking the details of its client so he could seek financial damages allegedly resulting from the allegations.

Supreme Court spokesman Steven Bakker said the court found Pessers' claim of having suffered damages sufficient to order Lycos to release its client's name and address, even though no criminal offense had been committed. It issued a sweeping rejection of Lycos's argument that personal client details should only be released if they are suspected of a crime and the information is wanted by the police.

"The court considers it probable that the information posted on the web site is illegal and damaging to Pessers," the ruling said. "Pessers has a genuine interest in obtaining the client's details and there is no other way to obtain them."

The Brain Institute, which represents the global entertainment industry in the Netherlands, said in a statement the ruling will enable it to seek damages from people who illegally swap copyrighted software, music and movies over the Internet.