Campaigners have lost their appeal at the UK Supreme Court over the right to die - but the judges said Parliament should now act.
Justices ruled against Paul Lamb and Jane Nicklinson by seven to two.
A third man, Martin, lost his attempt to have the current prosecution guidance on assisted suicide clarified.
But five justices concluded they had the power to declare the current law breaches the right to a private life - although they did not make a "declaration of incompatibility" in this case.
A majority of the justices said that the question they were being asked involved moral judgements rather than points of law - and the matter had to be addressed by a democratically-elected Parliament.
The cases involve the 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 wanted the law changed to allow doctors to assist patients to die.
Nine justices had to decide whether a prohibition on assisted suicide was compatible with the right to respect for private and family life under the European Convention on Human Rights.
Five concluded the court had the "constitutional authority" to make a declaration, and two of the five said they would have done so.
Such a declaration would not have struck down the law - but it would be a strong signal to Parliament that there needed to be change.
Mr Lamb and Mrs Nicklinson, the wife of Tony Nicklinson, said the conclusions were a "positive" step in the fight for change.
"I am very proud of myself," said Mr Lamb, 58. "I know it is going to change."
Mrs Nicklinson, from Wiltshire, whose husband Tony died aged 58 last year, added: "I am disappointed that we lost. But it is a very positive step. Parliament will have to discuss this. I think Tony would be very pleased at how far we have come."
Dominic Casciani, BBC home affairs correspondent
The Supreme Court is our highest legal authority - and that also makes it a political creature. So its right-to-die judgement boils down to a single phrase directed at Parliament: Sort it out, or we will.
It argues that MPs have ducked the issue for too long - and it is time they, as our democratically-elected representatives, find the political backbone to amend the law on assisted suicide.
Lord Neuberger, the court's president, clearly finds current guidelines, whereby prosecutors consider criminal charges after a death, to be deeply unsatisfactory.
He proposes a transparent legal channel which would allow independent assessment of a patient's application to die.
Lady Hale, an international authority on law and the family, is bolder, saying the ban breaches human rights.
So make no mistake: if Parliament doesn't act, it may only be a matter of time before judges run out of patience.
Commenting on the ruling, Andrea Williams of Christian Concern, said: "This is good news for the many vulnerable people who would have been at risk if the attempt to weaken the law on euthanasia and assisted suicide had been allowed by the Supreme Court."
Andrew Copson, chief executive of the British Humanist Association, which supported the campaigners, said: "It is clear that the Supreme Court went as far as it was able in urging Parliament to take action on the vital issue of assisted dying."
A third man, known only as Martin, was seeking clarification of the Director of Public Prosecutions's (DPP's) guidance on the position of health professionals who assist a suicide.
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 unanimously allowed the DPP's appeal against a previous Court of Appeal ruling in Martin's favour for clarification of the guidance.
Using an adapted computer operated by sight, Martin said in a statement: "I would implore the DPP to amend her written guidance as a matter of absolute priority.
"People in my situation, who have the right to choose how to end their lives, should be able to get the assistance they need from people outside their family."
Mr Lamb 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.
The 1961 Suicide Act makes it an offence to encourage or assist a suicide or a suicide attempt in England and Wales. Anyone doing so could face up to 14 years in prison.
But despite this, scores of UK citizens have travelled to Dignitas in Switzerland to end their lives - and no relative who has helped them has faced prosecution.
The law is almost identical in Northern Ireland.
There is no specific law on assisted suicide in Scotland, creating some uncertainty, although in theory someone could be prosecuted under homicide legislation.