A British doctor suffering from an incurable illness killed herself yesterday in Zurich with the help of Dignitas, the Swiss voluntary organisation.

Anne Turner was the 42nd Briton to seek medical help from Dignitas to end her life. Her case will cause controversy because she was diagnosed only last summer and as yet had relatively few symptoms of the brain disease, progressive supranuclear palsy (PSP).

Yesterday the UK organisation Dignity in Dying, which used to be known as the Voluntary Euthanasia Society, said Dr Turner's story showed British law was shortening lives and called for assisted suicide to be legalised.

"This case is truly heartbreaking," said its chief executive, Deborah Annetts. "The government must make time in parliament for the assisted dying for the terminally ill bill. Only this bill could have prevented Anne Turner from taking her life early. If this bill had been law, Anne would not have been forced to go to Zurich while she was still able to travel, for help to die. She would be alive today."

Dr Turner, who would have been 67 today, said in an interview shortly before her death that she did not want to reach the point where she could not travel to a country where assisted suicide is legal. "I think it's dreadful that somebody like myself has to go to Switzerland to do this, which is an awful hassle," she said.

"I want to go there while I still can, because I have to be able to swallow a solution of barbiturates. I know that people say that I look well, but I'm not. My condition has deteriorated an awful lot, particularly my speech. I hate talking on the phone now and, really, I don't like talking much at all."

Further evidence of Dr Turner's preparation and determination was apparent in the sheaf of envelopes she is seen clutching in the interview. She had prepared more than 100 typed letters for her family, friends and neighbours telling them of her plans. Each carried a personal, handwritten message on the back.

They were left in her kitchen and delivered to each recipient by her cleaner yesterday lunchtime.

The typed message in the letters read: "I have no shortage of people to convince me that life with the illness is worth living ... Because the law in the UK is not currently supportive, I have had extra complications which have required a certain degree of secrecy.

"I am firm in my belief that I must end my life now while I'm still able. I hope to have spared you some of the protracted grief."

Her son, Edward, 39, speaking to the Guardian from Zurich hours after her death, said the experience had been hard for Dr Turner's three children. "For us as a family it has been a really horrific experience and we did it for my mother," he said. "We wanted to help her in whatever way we had to." Dr Turner, who retired early from her job running a family planning clinic to nurse her husband, who had multiple system atrophy, would have had an average life expectancy of seven years with PSP, but would have faced the lingering death suffered by Dudley Moore, who died in 2002 of the same condition.

The rare disease causes degeneration of the nerve endings. Balance, vision, movement, speech and the ability to swallow all deteriorate. Death is usually triggered by something else, such as pneumonia or choking.

A former GP and campaigner for voluntary euthanasia yesterday told the Guardian that he watched a terminally ill woman from Glasgow take a lethal dose of barbiturates at the Dignitas clinic. Michael Irwin said: "I've never seen anyone actually commit suicide before. I wasn't sad or tearful. I was glad for her. This was a life she didn't want to lead any more." The 75-year-old widow from Glasgow, whose identity her family wants to keep secret, had multiple system atrophy and was in a wheelchair.

The dignity trade

Dignitas was founded by a Swiss lawyer, Ludwig Minelli, in 1998. The Zurich-based charity's aim is to help people with chronic illnesses to die a quick and painless death. Its motto is "Live with dignity, die with dignity." Patients die by drinking a lethal dose of barbiturate that kills them in about five minutes. The suicide is recorded on video to prove it is voluntary and the tape is examined by police and a coroner. Over the last eight years the organisation has assisted in the deaths of more than 450 people from around Europe, 42 of whom were from the UK. Although the law here prohibits assisted suicide, no one has yet been prosecuted for travelling to Switzerland to help a relative die.