The former leader of the Church of England George Carey on Saturday said he had changed his mind and would support a British bill to allow assisted suicide in certain cases.
Current Archbishop of Canterbury Justin Welby later restated his and the Church's opposition to the bill, saying it could open the way to abuse and neglect of older people.
But Dr Carey, who now sits in the House of Lords - parliament's upper chamber - after leaving office as the spiritual leader of the world's Anglicans in 2002, told the Daily Mail that he had dropped his long-standing opposition.
"The fact is I've changed my mind," he wrote in a piece for the British newspaper.
"The old philosophical certainties have collapsed in the face of the reality of needless suffering," he explained.
The former cleric, 78, said he would support the bill, brought by Charles Falconer, which would allow mentally-capable adults to request help to die if they were suffering from a terminal illness and had less than six months to live.
It is due to be debated in the House of Lords next week.
The Church of England has consistently argued for no law change, and its current leader repeated his opposition.
"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," Archbishop Welby wrote in the Times.
"It would be equally naive to believe, as the Assisted Dying Bill suggests, that such pressure could be recognised in every instance by doctors given the task of assessing requests for assisted suicide," he warned.