Should the law allow doctors to help terminally ill patients to hasten their deaths? That question is provoking heated discussion on both sides of the Atlantic.

More than 70 peers have lined up to speak in a debate today in the House of Lords which promises to stretch late into the night. Last week, in the first case presided over by the new chief justice, John Roberts, the Bush administration tried to persuade the US supreme court to allow the federal government to block physician-assisted suicide in Oregon, the only state so far to legalise it. The peers will debate a report on the assisted dying for the terminally ill bill, from a Lords select committee which visited Oregon, the Netherlands and Switzerland, and looked at the experience in Belgium.

The committee, headed by Lord Mackay of Clashfern, was set up to investigate issues surrounding the bill, introduced by the crossbench peer Joel Joffe, which ran out of time in the last parliament. It would have enabled "a competent adult who is suffering unbearably as a result of a terminal illness to receive medical assistance to die at his/her own considered and persistent request". The bill would have legalised physician-assisted suicide and, for patients physically unable to kill themselves, voluntary euthanasia.

The Lords committee was split on the issue and reached no conclusion on whether the law should be changed, merely reporting the evidence it had gathered, suggesting possible safeguards and calling for a debate as soon as possible.

Its report does, however, call for a clear distinction to be made in any future bill between voluntary euthanasia, where a doctor hastens death at the patient's request, and physician-assisted suicide, where the doctor helps the patient kill himself. That, it said, would help parliament to debate whether it wanted one, the other or both.

The legal position is different in each of the four countries which permit some form of help to end life. Belgium allows only voluntary euthanasia, not assistance with suicide. Switzerland allows anyone - not just a doctor - who acts from unselfish motives to help another to commit suicide. The Netherlands has legalised both voluntary euthanasia and doctor-assisted suicide. Oregon permits only the latter, under strict rules which require a prognosis of six months or less to live.

Lord Joffe is expected to introduce soon an amended bill along the lines of the Oregon law - dropping voluntary euthanasia, in his original bill - and legalising only physician-assisted suicide for terminally ill people.

A bill redrafted along Oregon lines would stand a better chance of getting through. Nine leaders from six faiths published an open letter last week arguing that the right to die would become the duty to die, with economic pressures coming to dominate decision making.

The government has adopted a neutral stance, as did the British Medical Association by a narrow vote last June, after refusing for years to countenance a change. A poll of 1,000 doctors in 2004 found that 56% favoured legalisation of physicianassisted suicide with stringent safeguards. Surveys of public opinion over two decades show growing support. In a YouGov poll last August, 86% agreed that people who are terminally ill "should have the right to decide when they want to die and to ask for medical assistance to help them".

The case of Diane Pretty, who died of motor neurone disease three years ago at the age of 43, attracted widespread sympathy for her plight. She fought an unsuccessful battle through the English courts and the European court of human rights for her husband, Brian, to be allowed to help her commit suicide before she faced the swallowing and breathing difficulties that come in the last stages of the illness. Announcing her death in May 2002, Mr Pretty said: "Diane had to go through the one thing she had foreseen and was afraid of and there was nothing I could do to help."

Opponents of change argue that good palliative care should avoid the need to undergo a painful death.

The law allows doctors to administer medication which incidentally precipitates a patient's death as long as the intention was not to kill but to relieve pain - the so-called "double effect". But the BMA debate last June acknowledged that "some patients have distressing symptoms at the end of life that cannot be completely alleviated by medical skills".

Oregon's experience offers some reassurance to those who fear that vulnerable elderly people would come under pressure. Only 208 people, most of them suffering from cancer, have chosen to kill themselves with a doctor's help since the law came into effect in 1997.