The Canadian researchers found that the "will to live" was unstable in cancer patients.

They fear that patients could opt for physician assisted suicide, or euthanasia, when at an extremely low ebb, when, had they waited even a few days, they would have made a completely different decision.

The recommendation is that, should voluntary euthanasia be legalised, patients should get a "cooling off period" in which they could change their minds.

The study, published in The Lancet, looked at 168 cancer patients who were receiving palliative care following a terminal diagnosis, assessing their mental state at regular intervals.

They found that a patient who strongly expressed the desire to die could say the opposite even some 12 hours later.

"The likely transience of a request to die is one of the most important considerations.

Euthanasia illegal in UK

"Demonstration of a sustained wish to die must be part of evaluating any death-hastening request."

Physician-assisted suicide, in which the doctor provides the means, or administers treatment to kill or hasten the death of a patient, is currently illegal in this country.

The policy of the British Medical Association (BMA) is also against physician-assisted suicide.

Its guidance states: "The normal duty of the doctor is to attempt to rescue the would-be suicide and considerable moral and practical confusion could be generated by the imposition of a duty to assist it."

The Voluntary Euthanasia Society said that it was entirely in favour of a 15-day cooling off period.

"It is an integral part of any proposal that we are making.

"This is a very much needed contribution to a much neglected field of study." Studies have already shown links between interest in physician-assisted-suicide and depression, pain and other distressing symptoms.

The researchers are hopeful that understanding this link could help provide better pain control and palliative care to cancer patients.