Nearly a third of those who are investigated for the "mercy killing" of a friend or relative with a terminal illness end up committing suicide because of the trauma of what they have done and the strain of society's reaction, it was revealed yesterday.

The Voluntary Euthanasia Society publishes the figures in a report today intended to highlight the difficult position faced by doctors and relatives when they are asked to help somebody to die.

"Behind the statistic of 30% of mercy killers going on to commit suicide, there lies a huge amount of suffering," said Deborah Annetts, chief executive of the society.

In spite of the much publicised deaths of Diane Pretty and Reginald Crew "the debate consists mainly of assumptions, and usually takes place in a vacuum without any reference to what really happens at the end of life," says the report.

Ms Annetts said legal uncertainties put terminally ill people and their relatives in great difficulties. Eleven out of 37 people who were investigated committed suicide. Seventeen were convicted of manslaughter and served between nine months and four years in jail. Five cases were dropped after advice from the director of public prosecutions and four were still pending.

England and Wales have the harshest laws on assisted dying in Europe, says the Quality of Mercy report. It quotes a 1998 survey which found that one in seven GPs in the UK admitted to helping people to die.