The U-turn was greeted as a major victory for the campaign against euthanasia and follows similar votes this year by the Royal College of General Practitioners and the Royal College of Physicians. Last month the House of Lords in effect rejected a Bill introduced by Lord Joffe that would have allowed terminally ill patients to end their lives, with the assistance of a doctor, in very limited circumstances.

The campaign for a euthanasia law had been given a significant boost last year when the BMA voted to move from clear opposition to a policy of neutrality for the first time in its 134-year history. But at the conference in Belfast yesterday, members voted by a 65 per cent majority against the legalisation of physician-assisted suicide and by the same majority against the legalisation of euthanasia. Physician-assisted suicide involves giving a patient the means to commit suicide, such as prescribing a lethal dose of barbiturates for self-administration. Euthanasia involves the active participation of the doctor.

Dr Peter Saunders, of the Care Not Killing alliance, said: "What we now have to do is to campaign to improve palliative care and educate the public to understand what such care can offer and that if a law came in, how vulnerable patients might feel under pressure to request an end to life."

Deborah Annetts, chief executive of Dignity in Dying, said: "Millions of people in the UK will be deeply disappointed."