Nearly one in three of those questioned thought it sometimes justifiable for a doctor to end a patient's life.

The overwhelming majority (83%) thought it acceptable to withhold life-sustaining treatment.

A similar number (86%) said that in some cases the desire to commit suicide was a rational act.

However, only one-third said they would be willing to assess patients to determine whether they were mentally ill at the time of making the request to die.

In total, 322 psychiatrists were asked for their views about end of life issues by a team of researchers from the Royal Free Hospital's department of psychiatry.

Caution over role

Psychiatrists do not want an active role

Study leader Dr James Warner said most psychiatrists, while supporting the right of a patient to die, were cautious about taking an active involvement in the process.

He said: "It places the psychiatrist in an ethically difficult position.

"They are trying to save life and to be involved in a process that is going to end a person's life could leave an uncomfortable taste in the mouth."

Dr Warner said psychiatrists were also too busy to take an active role in assessing patients who wanted to die.

"To assess whether a patient was mentally ill would not be something that could be done in an hour's consultation. It would require a very long assessment process, and psychiatrists do not have the time to do it."

Professor Simon Wesley, of King's College Hospital, London, a member of the Institute of Psychiatry, warned that a desire to die was often associated with depression, even among the terminally ill.

He said: "Even when someone was terminally ill a psychiatrist would want to know whether that person had a treatable mental disorder before the subject of euthanasia could ever be discussed."

Professor Wesley said most psychiatrists would be appalled by the idea that they could be asked effectively to screen people to die.