Lawyers for Internet service provider Totalise demanded financial sites Motley Fool and Interactive Investor International reveal the name of an anonymous user that commented on the company in discussion groups.
The user, known only as Zeddust, first aired his views on Motley Fool. When Totalise complained, the site removed the comments and banned Zeddust, but refused to reveal his identity. He then appeared on Interactive Investor, which followed the same course of action.
In his ruling, Mr Justice Robert Owen said the messages posted by Zeddust were clearly defamatory and that, unless he ruled for Totalise, people would be able to defame with impunity on-line.
In the past, firms such as ISPs have escaped censure because they acted only as carriers of data and did not edit the information they passed on. But Owen said this defence did not apply to Motley Fool because it monitored and managed discussions. Totalise is now expected to launch a libel action against Zeddust.
According to Nigel Miller, e-commerce partner at law firm Fox Williams, Owen's ruling reminds companies operating Web sites of their legal responsibilities.
"It puts on-line operators in a difficult position. Firms must update their terms and conditions to tell users that they cannot protect their anonymity if they break the law," he said.
But Miller pointed out that the order did not make the operators themselves liable for prosecution. "It's quite a sensible result that balances competing interests, " he added.