Desmond Tutu, one of the world's most eminent religious leaders, has made an extraordinary intervention in the debate over assisted death, by backing the right of the terminally ill to end their lives in dignity.

Writing in the Observer, the 82-year-old retired Anglican archbishop, revered as the "moral conscience" of South Africa, says that laws that prevent people being helped to end their lives are an affront to those affected and their families.

He also condemns as "disgraceful" the treatment of his old friend Nelson Mandela, who was kept alive through numerous painful hospitalisations and forced to endure a photo stunt with politicians shortly before his death at 95.

Tutu, who calls for a "mind shift" in the right to die debate, writes: "I have been fortunate to spend my life working for dignity for the living. Now I wish to apply my mind to the issue of dignity for the dying. I revere the sanctity of life – but not at any cost."

Tutu's intervention comes at the start of a momentous week in the assisted dying debate. On Friday, the House of Lords will witness one of the most significant moments in its recent history when peers debate an assisted dying bill proposed by the former lord chancellor, Lord Falconer. A record number of peers – 110 so far – have registered to speak.

On Saturday the former archbishop of Canterbury Lord (George) Carey spoke out in favour of the bill. But in an article in the Times, Justin Welby, the current archbishop and head of the Church of England, reaffirmed the church's traditional hostility to any move that would endanger the principle of the sanctity of life. In a sign of the debate that has now been unleashed within the Anglican communion, the bishop of Carlisle, the Right Rev James Newcome, called for a royal commission to examine the "important issue" at length.

Falconer's proposed legislation would make it legal for a doctor to hand over a lethal medication to a terminally ill patient who is believed to have less than six months to live.

Tutu notes that Falconer's bill will be debated on Mandela Day, which would have been the 96th birthday of South Africa's first black president. He calls for his own country to follow Britain's lead in examining a change in the law.

"On Mandela Day we will be thinking of a great man," he writes. "On the same day, on 18 July 2014 in London, the House of Lords will be holding a second hearing on Lord Falconer's bill on assisted dying. Oregon, Washington, Quebec, Holland, Switzerland have already taken this step. South Africa has a hard-won constitution that we are proud of that should provide a basis to guide changes to be made on the legal status of end-of-life wishes to support the dignity of the dying."

Speaking to the Observer, Falconer, who said he was now confident that his bill would live on in parliament beyond Friday's debate, claimed that the intervention by Tutu illustrated that religious faith should be no obstacle to supporting a change in the law. He said: "I am really glad that someone of his stature is taking part in this important debate. It is a debate in which countries look to other countries for guidance. For someone of Archbishop Tutu's stature, understanding and human experience to speak out is really welcome. He is an Anglican bishop who has shown his moral strength to the world better than anybody. I very much hope that it will indicate that religion is not a bar to supporting this bill."

A London rabbi, Jonathan Romain, speaking on behalf of 60 religious leaders in support of the Falconer proposals, said he believed that backing the bill was the "religious response" to a situation where medical progress allowed people to live on in a physical and mental state that many felt was intolerable. He said: "I see no sanctity in suffering, nothing holy about agony."

Jane Nicklinson, widow of the campaigner Tony Nicklinson, a sufferer of locked-in syndrome who fought for the right to be helped to die in the UK, said she believed public opinion was now in favour of change, adding: "I hope that it is true among those that matter – the decision-makers."

Falconer's proposals are being fiercely opposed by key figures such as Welby, and campaigners for the rights of disabled people. Richard Hawkes, chief executive of the disability charity Scope, said he feared the bill would put some people under pressure to end their lives. He said: "Why is it that when people who are not disabled want to commit suicide, we try to talk them out of it, but when a disabled person wants to commit suicide, we focus on how we can make that possible?"

However, in his article for the Observer, Tutu says that he has been moved by the case of a 28-year-old South African, Craig Schonegevel, who suffered from neurofibromatosis and felt forced to end his life by swallowing 12 sleeping pills and tying two plastic bags around his head with elastic bands because doctors could not help him.

Tutu writes: "Some say that palliative care, including the giving of sedation to ensure freedom from pain, should be enough for the journeying towards an easeful death. Some people opine that with good palliative care there is no need for assisted dying, no need for people to request to be legally given a lethal dose of medication. That was not the case for Craig Schonegevel. Others assert their right to autonomy and consciousness – why exit in the fog of sedation when there's the alternative of being alert and truly present with loved ones?"

He also discloses that he has now had a conversation with his family about his own death. "I have come to realise that I do not want my life to be prolonged artificially," he writes. "I think when you need machines to help you breathe then you have to ask questions about the quality of life being experienced and about the way money is being spent. This may be hard for some people to consider.

"But why is a life that is ending being prolonged? Why is money being spent in this way? It could be better spent on a mother giving birth to a baby, or an organ transplant needed by a young person. Money should be spent on those that are at the beginning or in full flow of their life. Of course, these are my personal opinions and not of my church."

There was bitter controversy in South Africa in April last year when President Jacob Zuma and other African National Congress politicians visited Mandela at his home with a TV crew. The statesman looked weak, rheumy-eyed and uncomprehending. Mandela's family and personal assistant condemned the publicity stunt as exploitative and in poor taste. Tutu echoes that view. "What was done to Madiba was disgraceful," he writes. "There was that occasion when Madiba was televised with political leaders, President Zuma and Cyril Ramaphosa. You could see that Madiba was not fully there. He did not speak. He was not connecting. My friend was no longer himself. It was an affront to Madiba's dignity."

"People should die a decent death," he continues. "For me that means having had the conversations with those I have crossed with in life and being at peace. It means being able to say goodbye to loved ones – if possible, at home."

He adds: "I can see I would probably incline towards the quality of life argument, whereas others will be more comfortable with palliative care. Yes, I think a lot of people would be upset if I said I wanted assisted dying. I would say I wouldn't mind, actually."

Tutu, who chaired South Africa's Truth and Reconciliation Commission, and admitted he was "angry with God" during apartheid, has never been afraid to take unpopular positions or stir debate. Mandela once said of him: "Sometimes strident, often tender, never afraid and seldom without humour, Desmond Tutu's voice will always be the voice of the voiceless."