The most dangerous stalkers are those who have had a sexual relationship with their victim, researchers have found.

Former partners are much more likely to physically attack their victims than psychotic strangers, a study of 50 stalkers at the Royal Free Hospital and University College Medicial School, London, found.

They are also less likely to suffer from a mental illness.

Dr Frank Farnham and his colleagues found that 14 of the 20 stalkers who had previously had a sexual relationship with their victim committed serious violence.

Among these cases there were five murders, three attempted murders, four cases of grievous bodily harm and two assaults causing actual bodily harm.

Case files

Only one in five of this group was diagnosed with a psychotic illness.

By comparison, only eight of the 30 stalkers who targeted total strangers committed violent physical abuse. Three-quarters of this group were diagnosed as psychotic.

The doctors identified stalkers by examining five years of case files from a north London forensic service.

Stalking was defined as "repetitive unwanted communications or approaches, which induced fear in the victim and were committed over a period of at least four weeks".

The 50 cases satisfied this definition. All the people studied had been arrested for criminal offences connected with their stalking behaviour.

'Not violent'

Dr David James, who helped carry out the new study, said: "The popular image of a stalker is that of the stranger in the night, something that is suddenly visited on you by some madman.

"What our research demonstrates is that in terms of serious physical violence, the most dangerous stalkers are those with whom the victim has been sexually intimate."

Dr James stressed that most people did not resort to stalking when a relationship broke down, and most stalkers were not violent.

He said: "When a relationship ends it can produce a lot of powerful emotions, feelings of loss, rage or anger.

"Obviously it is very common for people to plead on the phone and get upset.

"The difference in the behaviour we're looking at is that it instils fear.

"You have to be particularly insensitive and try particularly hard to risk being bracketed as a stalker at the end of a relationship."

Recent figures show that stalking is Britain's fastest growing crime.

Current estimates put the number of prosecutions under the Protection from Harassment Act, introduced three years ago to tackle stalking, at more than 4,000 a year.

This month Home Secretary Jack Straw authorised the launch of Britain's first national anti-stalking police unit.

The research is published in The Lancet medical journal.