Brian Pretty can not help his wife end her agony because of the 1961 Suicide Act, which bans assisted suicide, or so-called active euthanasia, no matter how awful the facts of the case.

The Prettys are challenging section 2.1 of the act, which reads: "A person who aids, abets, counsels or procures the suicide of another, or an attempt by another to commit suicide shall be liable on conviction, on indictment to imprisonment for a term not exceeding 14 years."

Mona Arshi, solicitor for Mrs Pretty, said: "If we win this case it won't open the floodgates. You'll be able to assist rational, competent adults who are terminally ill, and who can't do it themselves because of physical disability."

Victory for Mrs Pretty would inevitably reignite the wider debate about doctors being able to end the lives of patients in agony with their consent, and about living wills where a terminally ill patient expresses their wish to die in writing before their condition leaves them unable to do so.

Cases have emerged of doctors taking patients' lives and claiming they were acting to end suffer ing. Prosecutions and convictions have been patchy.

But in the past decade, a related area of the law has been altered and clarified to allow the passive ending of life through the non-continuation or withholding of treatment.

The most prominent case was that of Tony Bland, who was left in a permanent vegetative state by the 1989 Hillsborough disaster. The House of Lords allowed doctors to discontinue his feeding by a tube. Since then 20 more such patients have been allowed to die.

Doctors can also withhold resuscitation from seriously ill patients if their post-resuscitation quality of life would be poor.

What worries those who do not want change is doctors using any reform to kill patients for criminal motives or of the elderly be ing allowed to die against their wishes.

Dr Greg Gardner, of the Medical Ethics Alliance, said: "If an alleged 'right to die' became a precedent, medical resources could be denied to frail, vulnerable patients on the basis that they can opt to die."

But there is also the fear that patients who could possibly recover might be allowed to die. A second Hillsborough victim, Andrew Devine, was in a vegetative state for eight years before experiencing a partial recovery. He is now aware of his surroundings and can communicate with his family.

Only two places in the world allow voluntary euthanasia: the Netherlands, which became the first country in the world to legalise it, and the state of Oregon in the US.

In Oregon, doctors are only allowed to provide terminally ill patients with the lethal drugs to end their lives, and can not administer them.

Euthanasia is expected to be legalised in Belgium later this year, and Spain and Switzerland are considering laws to recognise living wills.