The House of Lords has begun debating Kim Leadbeater's bill to legalise assisted suicide after it passed the House of Commons in the summer.
Over 200 peers are expected to speak on the proposed legislation over the course of the two-day debate, which started on Friday and will resume next week.
Leadbeater's Terminally Ill Adults (End of Life) Bill proposes legalising assisted suicide for patients in England and Wales with a terminal illness and six months or less to live.
The bill passed its Third Reading in the Commons by only a narrow margin, and with considerably less support than at its Second Reading.
Given the narrowing of support since the bill was first introduced, opponents have continued to hope that the House of Lords will vote it down - something that remains a possibility as it is a private member's bill and was not promised in the Labour government's election manifesto.
Speaking on the eve of the debate, Christian social policy charity CARE urged peers to reject the "irredeemably flawed" bill.
CARE chief executive, Ross Hendry, said: “We recognise the difficulties peers are facing with this Bill and questions around whether the House of Lords can or should block it.
“Ultimately, our view is that at some point, Peers should vote this legislation down. It is both flawed and dangerous.
"Legalising this practice would send the harmful message that lives marked by illness and disability are not worth living.
"It could also open the door to abuses against vulnerable and marginalised members of society: lonely elderly people, disabled people, victims of domestic abuse and others."
He noted the concerns highlighted by a wide range of experts, including the Delegated Powers Committee which said this week that the bill "gives sweeping, unspecified and unjustified powers to the government".
A separate report published by the House of Lords Constitution Committee on Thursday said that the "degree of deliberation, assessment and scrutiny" was "significantly less than we would expect to see for an equivalent government bill".
"This is especially concerning given the subject matter of the bill," it said.
Mr Hendry added, “We call on peers to reject this irredeemably flawed bill and instead, put pressure on the Government to go much further on investing in palliative care.”
The Bishops of London, Newcastle and Southwark, and the Archbishop of York were expected to speak during the debate.
The Bishop of London, Sarah Mullally, said she would use the debate to express her “deep concerns” and “grave fears” about the bill.
The bishop, who is also the former Chief Nursing Officer for England, has consistently spoken out against the bill.
Speaking to the PA news agency ahead of Friday's debate, she outlined her "grave fears regarding its potential to disproportionately impact the most vulnerable in our society", and said that the "most crucial issue that needs addressing" was the quality and availability of palliative care in the UK.
