Rowan Williams, backed by the Archbishop of York and the 24 other Anglican bishops who sit in the House of Lords, will argue that legalising “assisted dying” would be a dangerous moral watershed for Britain.

Under the proposals, a mentally competent adult suffering from a terminal disease would be able to request medical assistance to die. The private members bill has been introduced by Lord Joffe, a retired human rights lawyer and crossbench peer.

Williams believes voluntary euthanasia would undermine the care of the sick and dying. During the second reading of Joffe’s bill this week, he will argue that the nursing profession opposes assisted dying and the majority of doctors feel it would damage their relationship with patients.

He tended his mother, Nancy, in “her last months of decline and dementia” and said: “When we see protracted pain and distress it stretches our compassion to its limits.”

But, he added, “this must not be an excuse for sleepwalking into a situation where the ordinary building blocks of trust . . . in the whole legal fabric of our society can be abandoned”.

The bishops of Oxford, Portsmouth and St Albans are among senior figures who will back the archbishop in the debate.

The Catholic Church in England has been campaigning against the bill and has urged members to write to MPs and peers expressing their opposition to voluntary euthanasia.

Lord Joffe says the campaign has turned nasty. He has received bags of hate mail including letters accusing him of being a Nazi and comparing his euthanasia bill to actions during the Holocaust.

“Malice and aggression pervades (some of) these letters without any wish by the authors to debate,” he said. “It is a matter of faith but there is no Christian compassion and plenty of blind hatred.”

The Church of England and the Catholic church have been backed by a powerful coalition of faith groups and medical associations called Care Not Killing. On Friday it will deliver a petition of 25,000 signatures to Downing Street objecting to voluntary euthanasia.

Also on Friday, a coalition of disabled rights activists called Not Dead Yet will begin an anti-euthanasia campaign. Some members have terminal illnesses and would be covered by the legislation.

Meanwhile, it emerged this weekend that a 58-year-old woman suffering from cancer and multiple sclerosis has become the latest Briton to travel to a Swiss clinic to commit suicide. Valerie Sliwinski, from Clacton-on-Sea in Essex, wanted to die because she was in constant pain.

She travelled with her son, Stefan, 34, to the Dignitas clinic in Zurich last Friday to end her life. She is believed to be the 43rd British patient to die at Dignitas.

Stefan Sliwinski has been questioned by Essex police. He said: “I did what I did only out of love and respect for my mother.

“I wish I could have made her better but there was no hope. She had no quality of life and was in constant pain and she wanted to die. I have nothing to hide.”

His mother died after doctors at Dignitas gave her a lethal dose of barbiturates.

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