A former Archbishop of Canterbury has come out in support of assisted dying.

The change of mind by Lord Carey of Clifton just days before a crucial parliamentary vote on the issue is particularly significant because he was regarded as a supporter of the conservative evangelical wing of the Church of England, which is fiercely opposed.

Lord Carey said tonight that he now supports helping some terminally ill patients to die.

"Anyone who has had to watch a loved one go through the final agonies of a painful terminal illness is bound to ask deep philosophical questions about the nature of life and death," he writes in The Daily Mail.

"I remember Dorothy, one of my congregation many years ago, at the beginning of my career, when I was a parish priest in Durham. An intelligent and faithful Christian lady, she had been an outstanding head teacher. In retirement, she found out that she had cancer in an advanced form. I visited her regularly in hospital. I saw the ravages of the illness on her body as she was given huge doses of morphine to try to keep the excruciating pain at bay.

"Talking about her discomfort one day as she lay with tubes dripping fluids in and taking fluids away, Dorothy questioned whether she could go on, whether life was worth living. 'It is quality of life that counts,' she whispered, 'not number of days.'

When I visited her again, I must have looked very miserable because she looked up at me and said: 'Why are you so sad, Vicar? I'm only dying, you know!"

Lord Carey, who was influenced by witnessing the plight of sufferers such as Tony Nicklinson, who campaigned in vain to be allowed the right to die, says: "Even the most devout believers will find their faith tested by the sight of a dying person in torment - especially when modern medicine could swiftly bring the torment to a merciful end.

"Indeed, this rapid advance of medical technology has brought us to a crucial ethical turning point.

"For, while drugs might be able to hasten the end more quickly and painlessly, sophisticated medical science also offers people the chance to be kept alive far beyond anything that would have been possible only a few years ago."

Next week, peers are due to debate Lord Falconer's controversial Assisted Dying Bill. The proposed new law would enable doctors to prescribe lethal doses of drugs to terminally ill patients who have stated a clearly expressed intention to end their lives.

Lord Carey says that until recently, he would have "fiercely opposed" the Bill.

"I would have used the time-honoured argument that we should be devoting ourselves to care, not killing. I would have paraded all the usual concerns about the risks of 'slippery slopes' and 'state-sponsored euthanasia'. But those arguments that persuaded me in the past seem to lack power and authority now when confronted with the experiences of those suffering a painful death."

He says the current law fails to address the fundamental question of why terminally ill patients should be forced to go on in unbearable pain and with little quality of life.

"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 will urge a change in the law in the Lords next week. "Today we face a terrible paradox. In strictly observing accepted teaching about the sanctity of life, the Church could actually be sanctioning anguish and pain - the very opposite of the Christian message. Indeed, there is nothing anti-Christian about embracing the reforms that Lord Falconer's Bill offers."

The Church of England, meeting in York this weekend, said in a statement: "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."

During the debate in 2012 Dr Rowan Williams, who succeeded Lord Carey as Archbishop, warned that changes to the law to allow assisted suicide would spell "disaster" and a shift in society's attitude to the sanctity of life.

The current Archbishop of Canterbury, Justin Welby, also supports current Church policy that the law should not change.

Rabbi Dr Jonathan Romain, chair of the Inter Faith Leaders for Dignity in Dying and editor of Assisted Dying - Rabbinic Responses, published this year, 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.

"There are also a growing number of clergy like myself who are only too familiar with those dying in pain, who see nothing sacred in suffering and for whom a religious response means allowing them the option of assisted death if they so wish.

"George Carey deserves praise for being brave enough both to re-examine previous religious certainties and to propose new approaches more appropriate for a changing world."

Lord Carey's views were in direct opposition to the current Archbishop of Canterbury, Justin Welby, who writes in The Times, in an article also published on his website that the idea that the only truly compassionate response a civilised society can make to those who are terminally ill and who wish to end their own lives is through allowing assisted dying "is both mistaken and dangerous; quite literally, lethally so."

He says the matter is of more than academic interest but is a matter of life and death.

"It is entirely understandable that when we see someone we love suffering we will suffer along with them and we will want to do almost anything to alleviate their suffering. All of us will have had some experience of that - some of us in extreme and terrible ways. In the last few weeks I have sat by the bedside of someone dying while unnecessary treatment was given. I have sat by the bedside of one of my own children, having to agree to treatment ending.

"Even in the face of such agony, I would make a plea that the deep personal demands of one situation do not blind us to the wider needs of others. There are many people whom we will never meet who face suffering every day of their lives. Among these are vulnerable people, often elderly or living with severe disabilities. Action on Elder Abuse, for example, states that more than 500,000 elderly people are abused every year in the United Kingdom. Sadly, the majority of such abuse and neglect is perpetrated by friends and relatives, very often with financial gain as the main motive.

"Compassion must be extended to these people when we consider changing the law to accommodate the smaller number of people who wish for help in ending their lives. If we are showing compassion only to those we know and love, there is a danger that it becomes a self-centred sentiment. True compassion suffers with all, including those whom we do not know or might never meet.

"It would be very naive to think that many of the elderly people who are abused and neglected each year, as well as many severely disabled individuals, would not be put under pressure to end their lives if assisted suicide were permitted by law."

He warns that he knows of health professionals who are already concerned by the ways in which their clients have suggestions "to go to Switzerland" whispered in their ears by relatives weary of caring for them and exasperated by seeing their inheritances dwindle through care costs. "I have received letters from both disabled individuals and their carers, deeply concerned by the pressure that Lord Falconer's bill could put them under if it became law.

"Compassion is not simply a feeling; it is a commitment to sharing in the suffering of others while trying to alleviate it. True compassion can be shown through care, through expending time and resources on those suffering and through offering hope even in the darkest of circumstances."