Former Archbishop of Canterbury Lord Carey has given a boost to the campaign for a change in the law on assisted suicide by announcing that terminally ill people should be able to exercise their right to end their lives.
The ex-head of the Church of England and spiritual leader of the Anglican Communion said he had dropped his long-standing opposition to legalising assisted dying.
Writing in the Daily Mail, he said it would not be "anti-Christian" to change the law.
Jane Nicklinson, whose husband Tony had locked-in syndrome and died almost two years ago aged 58, said she was "amazed and thrilled" at Lord Carey's U-turn on the issue.
Lord Carey's change in stance on whether to legalise assisted dying was, he said, inspired by Nicklinson's situation.
"It was the case of Tony Nicklinson that exerted the deepest influence on me," he wrote in the Daily Mail.
"Here was a dignified man making a simple appeal for mercy, begging that the law allow him to die in peace, supported by his family.
"His distress made me question my motives in previous debates. Had I been putting doctrine before compassion, dogma before human dignity?"
Reacting to the development Mrs Nicklinson told BBC 5 live's Stephen Nolan that her husband would have been happy.
"I'm amazed actually and thrilled because the Church has always been one of our greatest opponents," she said. "This is huge.
"I think Tony moved a lot of people but to hear he moved someone in such a prominent position - someone who is willing to come out and openly support our position - I'm just over the moon about it.
"I'm really pleased and I know Tony would be as well."
Nicklinson and paralysed former builder Paul Lamb lost a right-to-die fight in the UK's highest court last month, but at the time said they were hopeful that change would come.
Carey said he would be backing legislation tabled by Lord Falconer which proposes allowing doctors to prescribe a lethal dose to terminally ill patients judged to have less than six months to live. He warned that by opposing reform, the Church risked "promoting anguish and pain".
The intervention by Lord Carey, 78, marks an extraordinary U-turn by the cleric, who was Archbishop of Canterbury from 1991 to 2002.
He was part of a coalition which helped derail Lord Joffe's Assisting Dying for the Terminally Ill Bill in 2006 in the House of Lords.
Speaking at the time in the House of Lords, he said: "If introduced, assisted suicide might be treated as casually as abortion is today, after a few years."
His intervention comes as Lord Falconer's Bill will be debated on Friday in the House of Lords and as the Church of England's governing body started a crucial meeting at York University.
The Church of England has consistently argued for no change in the law on assisted suicide.
A statement said: "In February 2012 the General Synod passed a motion which 'affirms the intrinsic value of every human life and expresses its support for the current law on assisted suicide as a means of contributing to a just and compassionate society in which vulnerable people are protected'.
"The debate on the motion covered all of the issues raised by Dr Carey's article."
In his Daily Mail article, Lord Carey wrote: "The fact is that I have changed my mind. The old philosophical certainties have collapsed in the face of the reality of needless suffering."
He said the "rapid advance" of medical technology in keeping the seriously ill alive for many years is an "ethical turning point" and that the "mercy" offered through death could be "enshrined in law".
He added: "Today we face a central paradox. In strictly observing the sanctity of life, the Church could now actually be promoting anguish and pain, the very opposite of a Christian message of hope."
Under the 1961 Suicide Act, it remains a criminal offence carrying up to 14 years in jail to help take someone's life.
The Director of Public Prosecutions issued guidelines four years ago that made clear that anyone who assisted a loved one to die while "acting out of compassion" would be unlikely to be charged.
The Bill drawn up by Lord Falconer, a former Labour Lord Chancellor, which is modelled on the system in the US state of Oregon would mean patients would be able to administer a fatal dose of drugs to themselves, but would not be able to receive help if they could not lift or swallow it.
The process would have to be signed off by two doctors. The British Medical Journal published an editorial earlier this month backing the Falconer Bill.
But the British Medical Association (BMA) said its policy does not support a change in the law in assisted dying.
It added on its website that BMA policy is that doctor-assisted suicide should not be made legal in the UK.
Rabbi Jonathan Romain, of Maidenhead Synagogue, Berkshire, said: "The former Archbishop's words are like a breath of fresh air sweeping through rooms cloaked in theological dust that should have been dispersed long ago.
"He shows that it is possible to be both religious and in favour of assisted dying.
"It also indicates that the debate is not - as it often thought - a battle between the religious and secular camps, but is within the religious community too.
"There are many who have both a deep faith and a desire to see assisted dying legalised in Britain as an voluntary option for the terminally ill providing there are safeguards to protect the vulnerable."
The Rt Rev James Newcome, Bishop of Carlisle, told BBC Radio 4's The World Tonight: "The present law as it stands has just about the right balance between, on the one hand, real compassion for those who are suffering, and on the other hand, care for those who are elderly, disabled, and others who might find themselves in some considerable danger if the law was changed."
The Bishop also told the programme: "It has been the view of the Church of England that assisted suicide or euthanasia of any kind is not consistent with our beliefs, and I think that will continue to be the line that certainly those of us who are in the House of Lords and the bishops will be taking next Friday when this debated."