Right-to-die Supreme Court judgement due
The UK's highest court will give its judgement later on the cases of two severely disabled men who want others to be able to help them die.
It involves family of the late Tony Nicklinson, of Wiltshire, who had locked-in syndrome, and Paul Lamb, of Leeds, who was paralysed in a road crash.
They want the law changed to allow doctors to assist patients to die.
The Supreme Court judgement is expected on Wednesday.
The court has had to decide if the law prohibiting assisted suicide is incompatible with the European Convention on Human Rights because it denies Mr Lamb, and others like him, the right to choose the timing of their death.
Clive Coleman, BBC legal correspondent
The Supreme Court judgement is seen as perhaps the most ambitious attempt yet to change the law on the right to die.
Paul Lamb took up the legal challenge brought by the late Tony Nicklinson, who suffered from locked-in syndrome.
At a hearing at the Supreme Court last year his lawyers argued the current prohibition on assisting suicides interfered with his right to a private and family life, because it prevented the most severely disabled from getting medical assistance to end their lives at a time of their choosing.
Any doctor who helped someone to kill themselves would be guilty of assisting a suicide, a criminal offence that carries a maximum prison sentence of 14 years.
Subject to strict safeguards, including the medical assistance being sanctioned by a court, Paul Lamb wants any doctor who does assist a suicide of someone in his position, to have a defence in law.
The defence would be that of "necessity". However, many fear any change to the law on assisted suicide which would make the position of the sick, the infirm and the elderly, more vulnerable.
The fear is that members of these groups could easily feel themselves to be, or be persuaded that they are, a burden to others and decide to end their lives as a result.
A second man, known only as Martin, wants clarification of the director of public prosecutions's guidance on the position of health professionals who assist a suicide.
The existing guidance is unclear on whether they would be prosecuted.
There are nine judges on the panel, rather than the normal five.
Paul Lamb, 57, has been almost completely paralysed from the neck down since a car accident more than 20 years ago and says he is in constant pain.
He has called for the law to be changed so any doctor who helped him die would have a defence against the charge of murder.
Tony Nicklinson was paralysed from the neck down after suffering a stroke while on a business trip to Athens in 2005.
After losing his High Court battle last year, he refused food and died naturally, aged 58, a week later at his home in Wiltshire. His widow, Jane, is continuing his legal battle.
Earlier last year, Mr Lamb joined forces with Mr Nicklinson's family to fight a joint legal case.
In their Appeal Court case, the decision centred on whether the High Court was right to rule Parliament, not judges, should decide whether the law on assisted dying should change.
The three Court of Appeal judges unanimously dismissed the Nicklinson and Lamb challenge.
In the judgement, the Lord Chief Justice Lord Judge said Parliament represented "the conscience of the nation" when it came to addressing life and death issues, such as abortions and the death penalty.
"Judges, however eminent, do not - our responsibility is to discover the relevant legal principles, and apply the law as we find it," he said.
At the same hearing a third paralysed man won his case seeking clearer prosecution guidance from the director of public prosecutions (DPP) for health workers who help others die.
The man, known only as Martin, wants it to be lawful for a doctor or nurse to help him travel abroad to die with the help of a suicide organisation in Switzerland. His wife and other family want no involvement in his suicide.
The Supreme Court will also deal with the DPP's appeal against the Court of Appeal's ruling in Martin's favour.