Disgruntled fans of Sheffield Wednesday who vented their dissatisfaction with the football club's bigwigs in anonymous internet postings may face expensive libel claims after the chairman, chief executive and five directors won a high-court ruling last week forcing the owner of a website to reveal their identity.

The case, featuring the website owlstalk.co.uk, is the second within days to highlight the danger of assuming that the apparent cloak of anonymity gives users of internet forums and chatrooms carte blanche to say whatever they like.

In another high court case last week, John Finn, owner of the Sunderland property firm Pallion Housing, admitted just before he was due to be cross-examined that he was responsible for a website hosting a scurrilous internet campaign about a rival housing organisation, Gentoo Group, its employees and owner, Peter Walls.

Exposing the identity of those who post damaging lies in cyberspace is a growth area for libel lawyers.

Dan Tench, of Olswang, the law firm representing Gentoo, said: "This case illustrates an increasingly important legal issue: proving who is responsible for the publication of anonymous material on the internet. This is likely to be a significant issue in defamation cases in the future."

The website Dadsplace, set up to campaign against perceived injustices in the family courts, had a forum where anonymous postings made various accusations against Gentoo, Mr Walls and his staff.

Those posting the comments went to considerable lengths to hide their identity, and Gentoo's lawyers ran up a bill estimated to be about £300,000 - which Mr Finn will now have to pick up, along with any damages awarded - taking the case to court and amassing circumstantial evidence that he was behind the website.

Revealing the Sheffield Wednesday fans was comparatively easy since there was no secret about the website owner. The next move was to apply for a court order requiring him to reveal the identities of "Halfpint" and the other fans behind what the club's lawyers described as a "sustained campaign of vilification". Fans made serious allegations against the club's chairman, Dave Allen, and directors and shareholders.

The club's lawyers asked the judge, Richard Parkes QC, to order disclosure about the identity of 11 fans.

But the judge decided some fans, whose postings were merely "abusive" or likely to be understood as jokes, should keep their anonymity.

The judge ordered that three fans whose postings might "reasonably be understood to allege greed, selfishness, untrustworthiness and dishonest behaviour", should be unmasked. Their right to maintain their anonymity and express themselves freely was outweighed by the directors' entitlement to take action to protect their reputation, he said.

Court orders obliging websites to disclose the identity of users posting anonymous defamatory remarks began in 2001.

Dominic Bray, of K&L Gates, Sheffield Wednesday's solicitors, said: "There seem to be quite a lot of websites that are using their anonymity to make comments about people and think that there shouldn't be any liability for it. But the internet is no different to any other place of publication, and if somebody is making defamatory comments about people then they should be held responsible for it. What these cases do is just confirm that's the law - the law applies to the internet as much as it does to anything else."