London (AFP) - The death of a British woman accused of a vicious campaign of online abuse against the parents of missing girl Madeleine McCann has ignited debate over the growing scourge of Internet "trolls".

Brenda Leyland was found dead in a hotel room earlier this month after being confronted by Sky News over her alleged trolling of Kate and Gerry McCann, whose three-year-old daughter went missing in Portugal in 2007.

An investigation is ongoing, but has found no evidence of foul play or third party involvement.

Using the Twitter handle @Sweepyface the 63-year-old reportedly posted thousands of hate-filled messages about the couple.

Her name figured on an 80-page dossier compiled by members of the public cataloguing alleged abuse directed at the couple and their two other children from a long list of Internet users, which is currently being investigated by the police.

It is a trend that has been replicated the world over against high-profile figures.

Zelda Williams, the daughter of US actor Robin Williams, recently quit Twitter after Internet trolls posted fake photos claiming to be her dead father.

Former model Charlotte Dawson, who was found dead at her Sydney apartment in February after battling depression, had been subjected to a torrent of abuse on Twitter.

- Global challenges -

"Every country is facing these challenges," said British lawyer Mark Stephens.

"What they are doing is meeting the challenges in slightly different ways but ultimately very similar balances are being struck."

The media lawyer said he had seen "a significant upswing" in online bullying cases.

But criminal prosecution, said Stephens, should be reserved for the most extreme cases.

"It is only a very small minority who are fixated, who take it to the extreme -- people who are borderline certifiable," he argued.

Britain's Crown Prosecution Service (CPS) has instructed lawyers that messages sent via social media could be a criminal offence if they contain "credible threats of violence" or target an individual in a way that "may constitute harassment or stalking".

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"Grossly offensive, indecent, obscene or false" messages could also amount to a crime if a "public interest" case can be made.

These guidelines, introduced against a background of mounting confusion about such cases, have been shaped considerably by public opinion, said Chris Holder, of London law firm Bristows.

"I think people are so fed up with reading things on the Internet that you would not say to people's faces -- abusive messages to people's children, Facebook posts that are completely obnoxious. The public have demanded that the CPS do something about it," he said.

He acknowledges, however, that in the fast moving world of the Internet any guidelines "could become irrelevant".

- Locking up 'trolls' -

Over the past decade the number of successful prosecutions for communications offences has risen in Britain and custodial sentences are not uncommon.

Last month a man was jailed for 18 weeks for what British prosecutors described as "a campaign of hatred" against a female lawmaker.

Peter Nunn, 33, from Bristol, southwest England, bombarded Stella Creasy with abusive tweets after she supported a campaign by feminist activist Caroline Criado-Perez to put the image of novelist Jane Austen on the Â£10 note.

Nunn claimed he sent the messages, which included rape threats, to exercise his right to freedom of speech and "satirise" online trolling.

Barbora Bukovska, a senior director at ARTICLE 19, an international organisation which promotes freedom of speech, said criminal sanctions should be "the last resort".

Bukovska said she did not defend trolling and admitted that some things posted online were "disgusting" but asked: "Do we want to criminalise every social conduct that we find problematic?"

"If you prohibit any harsh or offensive communication then it can be taken to the extreme," she said.

What drove the outpouring of bile about the McCanns by Leyland, a church-going mother-of-two in a sleepy English village, remains a mystery.

However, new research confirms what many victims already know, that online trolls can be a sinister bunch.

A study by Canadian researchers cited in Psychology Today linked trolling to sadism.

"Both trolls and sadists feel sadistic glee at the distress of others. Sadists just want to have fun... and the Internet is their playground!" it said.