A terminally ill cancer sufferer has become the first Briton to take his own life at an 'assisted suicide' clinic in Switzerland, The Observer can reveal.

The move has dismayed campaigning groups who have condemned the growth of 'death tourism' and warned of the risks of prosecution for the families of the dying. The children of the unnamed 77-year-old, who had throat cancer, could now face charges because they accompanied him on his trip. Though the suicide occurred abroad, it is still illegal under British law to help someone take their own life.

Pro-euthanasia groups have called for a change in British law. The Voluntary Euthanasia Society warned that 'death tourism' could result in dozens of Britons going abroad to commit suicide without proper safeguards being in place. 'This is a terribly sad situation,' said Deborah Annetts, chief executive of the society, which campaigns to allow doctors to assist in the deaths of the terminally ill. 'We hear from desperate terminally ill adults and their families on a daily basis. They are forced to consider taking drastic measures because the law in the UK prevents them from requesting a good death.'

The Swiss clinic is run by Dignitas, a local group that organises assisted suicides. The Observer has learnt that the British man flew to Zurich on Wednesday 23 October, accompanied by his son and daughter. At a small hotel in the city he met Ludwig Minelli, the Swiss lawyer who heads Dignitas, then went to meet a local doctor who works for the group. The doctor spent an hour examining the man's medical records, and talking to him to ascertain that he was in a fit mental state to make the decision. On Friday afternoon, after spending the morning together in the city, the British family arrived at a small suburban flat rented by Dignitas. 'The man was very calm, very peaceful,' said Minelli. 'Clearly, he had thought about this course of action very hard before embarking upon it.'

In the early afternoon the man was given a barbiturate to swallow. 'This is very quick and painless,' said Minelli. 'After five minutes, the patient fell asleep and went into a deep coma from which he did not wake. He would not have suffered any negative sensation.'

Cremation took place in Zurich and the family returned with his ashes to the Britain. Their identity has not been revealed as they want to maintain their privacy.

The family paid no fees as the service is paid for from Dignitas's membership fees of £10 per person. In the past four years the non-profit making organisation has helped 134 people end their lives. So far 14 British people have become members of Dignitas. Minelli said dozens of others had expressed interest. 'We will help these people, but really they should be able to have an assisted suicide in their own country. It is wrong that someone who is very ill should have to leave his home and his country and fly thousands of miles in order to be assured of a peaceful death.'

But many feel that allowing assisted suicide is a 'slippery slope' which could result in some curable patients dying. The British Medical Association is against any such measure because it says it would alter the balance of trust between doctor and patient.

Authorities in Switzerland are deeply worried about the influx of foreigners to the Zurich-based clinic and are considering passing emergency legislation to prevent this. Zurich public prosecutor Andreas Brunner said: 'Death tourism is a very touchy subject. What if someone has psychological problems? What if they have an illness which it turns out, is actually curable?'

The death of Diane Pretty earlier this year raised the issue of assisted suicide in Britain. Despite a long and courageous campaign Pretty, who had motor neurone disease, failed in the European Court of Human Rights to win the right to end her life with the help of her husband Brian. She died of natural causes in May this year.