The cross-party campaign to legalise the right to die took a significant step forward last night when the former archbishop of Canterbury, Dr George Carey, announced his support for the proposal as a way of preventing "needless suffering".

As peers prepare to debate a bill next Friday to legalise assisted dying, the former head of the worldwide Anglican church said it would not be "anti-Christian" to ensure that terminally ill patients avoid "unbearable" pain.

The intervention by Lord Carey of Clifton, 78, who served as archbishop of Canterbury between 1991-2002, could change the terms of the debate over the bill tabled by the former Labour lord chancellor Lord Falconer of Thoroton.

The assisted dying bill, due to be debated next Friday at second reading in the House of Lords, would legalise assisted dying for the terminally ill in England and Wales.

The Church of England is strongly opposed to the bill. But in an article for the Daily Mail, Carey said he had changed his mind after witnessing the pain of Tony Nicklinson who suffered from locked-in syndrome. He died two years ago just weeks after losing his high court battle.

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.

"It was the case of Tony Nicklinson that exerted the deepest influence on me. 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?

"I began to reconsider how to interpret Christian theology on the subject. As I did so, I grew less and less certain of my opposition to the right to die."

The Falconer bill would allow doctors to administer a lethal dose of drugs to terminally-ill patients with less than six months to live who have the mental capacity to make an informed choice. The patient's condition would have to be assessed by two doctors.

The former archbishop wrote of how he challenged his own thinking as he re-read the Scriptures. He wrote: "One of the key themes of the gospels is love for our fellow human beings ... 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."

Carey added that advances in modern medicine has been a crucial factor in his thinking. "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. Yet our laws have not caught up with the science.

"Until recently, I would have fiercely opposed Lord Falconer's bill, following the traditional line of the Christian church. 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 which persuaded me in the past seem to lack power and authority when confronted with the experiences of those approaching a painful death.

"It fails to address the fundamental question as to why we should force terminally ill patients to an unbearable point. It is the magnitude of suffering that has been preying on my mind as the discussion over the right to die has intensified."

Carey added that the Falconer bill would take mercy killings out of the "legal twilight". He wrote: "It is a shameful blot on our country's great reputation for caring for others that we have not come up with a better alternative than the Zurich clinic.

"Then there are all those cases conducted in the shadows, where doctors, friends or relatives privately carry out mercy killings with a high dose of drugs."

Rabbi Dr Jonathan Romain, the chair of Inter-Faith Leaders for Dignity in Dying, welcomed Carey's intervention. Romain 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."

MPs and peers will be given a free vote on the bill. Norman Lamb, the Liberal Democrat care minister, is expected to support the measure.