The libel laws are in need of reform to cope with the new menace of "cyber-vigilantism", lawyers said yesterday.

In one case that is the talk of internet chatrooms, a teenager has been publicly humiliated and vilified after incurring the fury of a buyer who bought his laptop and then claimed it was defective.

The buyer wreaked his revenge by setting up a fake website apparently in the name of the Amir Tofangsazan, using personal information as well as pornographic images that the buyer claims he found on the computer hard-drive. The site is being investigated by police.

It has registered more than half a million hits and, Mr Tofangsazan says, made his life into a "living hell".

At the same time, a site which allows women to vent their spleen over cheating lovers is booming in popularity. The American website Don’tDateHimGirl.com is awash with postings warning women against the alleged cheaters in their lives.

One posting shows a photograph of a man from the North of England who has a fiancee of 20 years and allegedly has been "having an affair with a married woman for several years and more recently with another woman."

It says: "He frequents internet dating sites looking for "the one" but is really only interested in dirty chat and one-off meetings for sex." The site has had 16,000 page views.

Another posting on the same site names another man and warns: "Ladies, watch out for this one."

"He’s bipolar, mean-spirited, verbally abusive, compulsive, very big drinker (which is probably the cause for a lot of his issues). Impotence being one of them."

The sites gives age, height and brief description. It also invites the men to reply.

Mark Stephens, the media lawyer from Finers Stephens Innocent, said: "This kind of cyber-vigilantism is likely to end up in our libel courts in vain attempts to remove the stain of false allegations and misidentifications as this spreads across from America".

But the sites also provided valuable consumer information. "There may also be a case for reviewing the libel laws as a result of sites like this."

But in the case of Mr Tofangsazan, he said that if the posted information was true, it could not be libellous - especially as it could be argued that the seller had himself put the information in the public domain by sending out the computer in the first place.

The mocking first-person blog about Mr Tofangsazan begins: "Hello. My name is Amir Massoud Tofangsazan and I live in Barnet. I’m 19 but pretend to be a lot older and like to pretend that I’m a big businessman when I’m not actually that clever."

It suggests that Mr Tofangsazan sold his broken laptop computer for £375 on eBay, and goes on to show pictures of his family and friends, a copy of his passport, and pornography the buyer claims to have found on his hard drive.

There are also pictures of him with a bare chest, with a drawn-on moustache and devil’s horns, kissing a girl, and sleeping, as well as pictures of women’s legs apparently taken on the London Underground.

Mr Tofangsazan told a newspaper: "I am shaking all over and I fear my reputation is going to be ruined. They have put photos of me and my family on there after hacking into my hotmail account, as well as publishing personal details and bits of my CV.

"There are photographs on there from the September 11 bombings, porn pictures and pictures of women’s legs which have absolutely nothing to do with me. Some people even think I took them.

"The laptop wasn’t even broken. It was in working order. The last few days have been a nightmare, some of my friends have seen it and my father is very angry."

A Scotland Yard spokesman said: "We have got an allegation of improper use of public electronic communications network that was reported to us on May 29 in person at Fulham police station.

"The allegation follows from a civil dispute. No arrests have been made and inquiries continue. It will be investigated by Barnet police."

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