Doctors from the British Medical Association have changed their position on the idea of helping patients to die.
End-of-life issues stimulate strong opinion
In a narrow vote last year, the BMA adopted a neutral stance on euthanasia and physician-assisted suicide.
The decision has now been overturned after 65% of the 500 doctors at the BMA's meeting in Belfast voted against assisted dying.
A bill to relax current law was blocked by the House of Lords in May, but is likely to be reintroduced.
The bill, sponsored by cross-bench peer Lord Joffe, would give doctors the right to prescribe drugs that a terminally ill patient in severe pain could use to end their own life.
WHAT THE TERMS MEAN
Assisted dying - a physician prescribes medication which a patient can take to end their own life
Voluntary euthanasia - the physician would actually help the patient die
Last year the BMA conference agreed that the issue of assisted dying was primarily a matter for society and parliament.
However, many doctors were unhappy at the vote, remaining implacably opposed to any form of assisted dying.
They argued that improvements in palliative care meant that even the most stricken of patients could be helped effectively through their final days.
In a heated debate, doctors argued for and against.
Dr John Fitton, a GP from Kettering, said despite the "blanket cliche" of good palliative care "people still die in undignified misery".
But Dr Andrew Davies, from Cardiff, said terminally ill patients in his care had "a lot on their minds" but for many, their main concern was the effect their illness was having on their families.
"My worry is that a right to die will become a duty to die, a duty to unburden their families."
The vote comes as a survey of 200 doctors, carried out by GFK Healthcare for Dignity in Dying, found 30% of GPs would be willing, in principle and if the law permitted, to write a prescription to assist a patient to die if their suffering could not be relieved by palliative care.
Deborah Annetts, chief executive of Dignity in Dying, said the vote came after intensive lobbying by religious lobby groups.
"Millions of people in the UK will be deeply disappointed."
Liberal Democrat MP Dr Evan Harris, a former hospital doctor who proposed last year's motion, said: "Doctors are split and at the moment the religious lobby is winning the tactical battle, but society should not allow religious views on the sanctity of life to trump the right to autonomy of a patient who does not share those views."
Dr Peter Saunders, general secretary of the Christian Medical Fellowship, denied claims of undue influence, saying just 10 to 12 members were attending the BMA conference.
And he added: "If good palliative care is provided, requests for euthanasia are extremely rare.
"We should be doing all we can to make sure that this care is made more widely available."