There are concerns about information accessible on the web

Dr Sundararajan Rajagopal said a disturbing new trend in suicide pacts involving strangers who met on the web was emerging.

In October, nine people in Japan died after meeting over the web and using special sites to plan their deaths.

Dr Rajagopal, of London's St Thomas' Hospital, made his comments in the British Medical Journal.

He said traditional suicide pacts account for less than 1% of all suicides, and almost always involve people well known to each other.

FOLIE A DEUX Suicide pacts are often associated with a rare psychiatric disorder called folie a deux Two people share the same or similar delusional beliefs The relationship among people with this disorder is also usually enmeshed and isolated from the rest of society

Most are spouses, and most of them are childless

About half have psychiatric disorders and a third have physical illnesses.

However, there is a risk that the internet is helping to break this mould by fuelling a phenomenon dubbed cybersuicide.

An increasing number of websites graphically describe suicide methods, including details of doses of medication that would be fatal in overdose.

Dr Rajagopal said such websites can perhaps trigger suicidal behaviour in vulnerable people - particularly adolescents - by giving deeply depressed people an opportunity, which they might not otherwise have, of getting in touch with others who feel the same way.

He said the recent suicide pacts in Japan might just be isolated events in a country that has been shown to have the highest rate of suicide pacts.

However, it is possible they might herald a new disturbing trend.

Dr Rajagopal said that if the web was increasing the risk of strangers making suicide pacts then it was most likely to impact on young people living alone.

He said GPs and psychiatrists should be aware of the potential risk, and ask patients whether they have used the web to obtain information about suicide.

In total, 26 people in Japan are thought to have died in suicide pacts in less than two months. In the latest incident seven people were found dead on Monday.

Two girls in South Wales arranged a suicide pact after becoming friends via an internet chat room.

Rebecca Ling, 14, survived the overdose attempt while her best friend Laura Rhodes, 13, died earlier this year.

Not all bad

Sarah Nelson, a spokesperson for the Samaritans, told the BBC News website there was no doubt that the web had an impact on people's behaviour.

But she said suicide was a complex issue, and more work was required to pin down exactly what sort of impact the internet was likely to have in this area.

"It is too simplistic to condemn the web," she said.

"There is no doubt it can be a positive force, for instance by allowing people to discuss their emotions.

"But any medium which publishes explicit details of suicide methods is likely to encourage some people to act.

"It is also not a very positive thing for suicidal people to be counselling others in the same position."