A woman with multiple sclerosis is facing an "impossible dilemma" over ending her life because of confusion over the law, Law Lords have been told.
Debbie Purdy, from Bradford, plans to go abroad to end her life but fears her husband may be prosecuted if he helps.
It is illegal to assist a suicide, but no-one has been charged over helping someone go to Swiss clinic Dignitas.
Her legal team said that if the law was not clarified she would have to end her life earlier than she wanted to.
The two-day House of Lords hearing, which started on Tuesday, represents the 46-year-old's last chance in the UK legal system.
She has already lost High and Appeal court cases and if she loses this challenge the only option open to her would be to go to the European Court of Human Rights.
Ms Purdy was diagnosed with primary progressive MS in 1995 and is now losing strength in her upper body. She has been in a wheelchair since 2001.
Lord Pannick QC, who is representing Ms Purdy, said both she and her husband wanted to know whether he was likely to be prosecuted and what criteria the Director of Public Prosecutions (DPP) takes into account when deciding whether to bring a charge under the Suicide Act.
"While this will not provide a guarantee that the appellant's husband will not be prosecuted it will enable the appellant to make a better-informed decision.
"If the risk of prosecution is sufficiently low, she can wait until the very last minute before travelling with her husband's assistance."
He said if the risk was high, she would have to go earlier while she was still fit enough to travel without assistance.
"It is ironic that the policy designed to protect the sanctity of life will have the effect of shortening the life of terminally-ill persons such as Ms Purdy."
He added this amounted to an "impossible dilemma".
The hearing comes ahead of an attempt later this week by the former Lord Chancellor Lord Falconer to change the law.
He will table an amendment to the Coroners and Justice Bill to lift the threat of prosecution facing those who help somebody kill themselves overseas.
Lord Falconer said it was probably a crime under the current law to assist somebody to go to a clinic abroad - but nobody was usually prosecuted.
He said: "Debbie is right to say that that is a very uncertain position."
Ms Purdy is also being supported the Dignity in Dying campaign group.
Chief executive Sarah Wootton said: "Parliament urgently needs to acknowledge the fact that people are travelling overseas to die - and this trend show no sign of stopping."
But not everyone agrees.
Dr Peter Saunders, of the Care Not Killing Alliance, which represents a range of organisations including disability and Christian groups, said: "The current law is clear and acts as an effective deterrent to those who seek to push its boundaries whilst also giving scope for judges to temper justice with mercy in hard cases.
"To force the DPP to offer case-by-case guidance ahead of time would turn it into a consultancy service for law-breakers and would inevitably lead to instances of abuse."
It is not the first time the issue has been raised in the courts.
In 2001 Diane Pretty, who had motor neurone disease, failed to get immunity from prosecution for her husband if he helped her to die in the UK.
Several attempts to legalise suicide in Britain have also been rejected.
In England and Wales, aiding or abetting a suicide is a crime punishable by up to 14 years' imprisonment.
It is expected to be several weeks before a ruling is made public.