As figures show 13,000 patients want a right to die, Dr Nigel Cox talks for the first time about his role in a mercy killing

Dr Nigel Cox still thinks affectionately of the patient he killed. The remains of Lillian Boyes are buried in a graveyard 200 yards from his home. "She was a plucky, game lady," he said of the woman whom he lethally injected after she begged him to put her out of her pain once and for all.

Nor can the consultant ever forget the repercussions of what he did: a conviction for attempted murder, being hauled up in front of the General Medical Council and a blighted career. But he does not regret his decision.

"Most doctors will deny it if you ask them 'Have you ever done it?'" said Dr Cox, the only doctor ever to be convicted in the UK of attempting to perform a mercy killing.

"But most doctors, among themselves, will admit that they have bumped off two or three patients at their request. And, certainly, where I have seen this happening, I've never had any doubt that it was the correct thing to do."

In an exclusive interview with The Independent on Sunday, the consultant, who still practises, spoke for the first time as peers urged parliament for a debate on whether terminally ill patients should be given the right to die, amid reports that there could be up to 13,000 such cases in the UK each year.

The Bill aimed to legalise assisted suicide - where the doctor provides a lethal prescription and the patient chooses whether to take it. And voluntary euthanasia - where the doctor is responsible for both prescribing and administering the fatal dose. The patient would have to be terminally ill, mentally competent and suffering unbearably. Lord Joffe plans to introduce a new Bill in the next parliament.

Dr Cox, a consultant rheumatologist at Winchester's Royal Hampshire County Hospital, had been caring for Mrs Boyes for 13 years, when, in August 1991, the 70-year-old pleaded with him to end her life.

Mrs Boyes, from Bishopstoke in Hampshire, was suffering from rheumatoid arthritis. She was in such acute pain she couldn't bear to be touched, and, according to one of her nurses, screamed like a wounded dog.

The hospital chaplain, the Rev Robert Clarke, said: "When anyone touched her you could hear the bones move about in their joints. The sound will stay with me to the grave."

Dr Cox, 59, who still lives in Colden Common, Hampshire, had tried all he could to relieve the agony. "What we were doing seemed to be making her worse. We were giving her huge doses of heroin."

He administered two ampoules of potassium chloride, with the aim of stopping her heart. After she died, Patrick, one of her sons, shook Dr Cox by the hand and thanked him.

"I don't know by how much I shortened her life," Dr Cox said. "But something between 15 minutes and an hour."

Dr Cox's undoing was to enter in the hospital log the exact dose he used - twice that needed to cause death. It was noticed by a nurse who, after agonising over a weekend, reported it. "I recorded it because she was a remarkable lady and it seemed inappropriate to end her life on a lie," he said. On the death certificate the cause of death was "bronchial pneumonia".

Charged with attempted murder, he was suspended for 18 months, but allowed to teach at another hospital and continue his private practice. "You foul up and think shit, you grit your teeth and you've got to get through it," said Dr Cox, who denied the charge.

He was convicted in September 1992 and given a 12-month suspended sentence. Mrs Boyes's sons were horrified.

In November 1992, the General Medical Council's professional conduct committee decided no further action would be taken. Dr Cox returned to work the following February under supervision.

"It's a very horrendous event being arrested and suspended and tried for murder. It clearly marks you to some extent. I'm sure it has changed me. It's possibly made me more cautious. I know two deaths shortly after that I cocked up, and patients did not get the pain relief they should have. They died a more painful death." How did he feel about that? "Not good," he said bleakly.

"Should I have done anything different? I don't think I should. But if you ask me whether I would do it again, in similar circumstances, I don't know. I might not have the balls to," he said.

He also revealed most doctors had killed a patient who had asked to die by giving them large doses of an opiate. "I have never known it happen where I didn't think that was the good and appropriate thing to do."

His belief is supported by a detailed survey of 1,000 British doctors which revealed last year that 45 per cent think their colleagues could be helping patients to die or commit suicide.

Speaking on condition of anonymity, one retired doctor told The Independent on Sunday last week how he had helped two patients die with lethal doses of morphine. Both were in the last ravages of terminal cancer.

"I have no qualms at all," he said. "I think the basis of our ethos is to relieve suffering, not prolong life beyond a ridiculous point."

Seeing such patients suffer, he said, was "horrendous". "It's the most unpleasant thing seeing somebody gradually fade away. I hope that if I ever reach that stage, someone will do the same for me."

Dr Cox, who still has the odd beer with John Boyes, welcomes a change to the law. "I think the Bill should be there with very well defined ways to make sure it wasn't abused.

"There are some people for whom their pain isn't properly controlled, and they die horrible deaths.

"If they are being properly managed, I think it is reasonable to ask for help to end their days."