We will appeal if we lose because I want her to have her final wish

Brian Pretty

Ms Pretty will argue that her quality of life is so low that she has the right under human rights legislation to chose to die.

She will lodge papers at the High Court challenging the Director of Public Prosecutions' (DPP) refusal to rule out prosecuting her husband Brian if he helps her commit suicide.

Prosecution risk

Her case is backed by the Voluntary Euthanasia Society and civil rights group Liberty.

Ms Pretty says that the DPP's decision last week will subject her to inhuman and degrading treatment, in contravention of the Human Rights Act.

Dianne Pretty is beginning her legal battle

Her husband Brian told the BBC of their determination to fight for a change in the law.

He said: "This is against the law at the moment and there is no way we are going to go against the law.

"So we will go to court. We shall fight in court and we will appeal if we lose because I want her to have her final wish."

His wife of 25 years was diagnosed with motor neurone disease in 1999, and her condition has deteriorated rapidly.

Mr Pretty said: "She is unable to walk, she is unable to use her hands or arms, and her speech is very limited. The easiest way to describe it is being like a baby, but with the knowledge and thoughts of a mature person."

Terrible suffering

Supporters of her case, including her children, Clara, 24, and Brian, 22, say she is clear about her decision, but physically unable to take her own life without assistance.

Dianne Pretty with husband Brian on their wedding day 25 years ago

She wrote to the prime minister earlier this year in a bid to have the law on voluntary euthanasia changed.

Last month, Liberty asked the DPP to guarantee her husband would not be prosecuted for aiding and abetting a suicide under Section 2 of the 1961 Suicide Act if he tried to help her.

Although DPP David Calvert-Smith conceded Mrs Pretty and her family were having to endure "terrible suffering", he said he could not offer such a guarantee. Liberty will challenge the way the DPP has applied the Suicide Act to the case.

John Wadham, director of Liberty, said: "You or I could make a decision to commit suicide, she can't because physically she is unable to do so.

"We are saying she should be put in the same position as anyone else, and that means her husband having to help her."

Deborah Annetts, of the Voluntary Euthanasia Society, said: "She is fighting not just for herself, but also for other people in similar cirucmstances.

"She knows that her days her numbered, but she really does want to change the law for others who come after her."

Medical opinion

It is always very difficult to tell after the event if somebody genuinely wanted to die or whether in fact they were killed

Dr Vivienne Nathanson

Dr Vivienne Nathanson, head of the ethics for the British Medical Association, sympathised with the Pretty family, but said the case was unlikely to be successful.

She said: "The law in the UK says you cannot help people to die and it equates this with murder.

"It does that for good reason because it is always very difficult to tell after the event if somebody genuinely wanted to die or whether in fact they were killed."

Dr David Oliver, a consultant in palliative care, said medical advances meant that people with motor neurone disease could die peacefully without the need for their lives to be ended prematurely.