The Anglican and Catholic churches say proposed Tasmanian voluntary assisted dying legislation "normalises suicide" and "threatens the lives of the vulnerable".
The Office of Parliamentary Counsel is drafting legislation to be introduced into the Legislative Council next year by the independent member for Mersey Mike Gaffney.
Anglican Dean of Hobart the Very Reverend Richard Humphrey said there was no "more serious ethical decision" for Parliament to consider than that raised by the proposed legislation.
He said the sanctity of life was "not simply a religious statement, it is a foundational marker of the legal framework of our society".
"To legalise assisted suicide is in direct contradiction to our cornerstone value of protecting human life," Reverend Humphrey said.
"Legalising euthanasia is assisted suicide and so 'normalises' suicide.
"This conflicts with the public health message conveyed through suicide prevention programs."
Reverend Humphrey said vulnerable people would be at risk from voluntary assisted dying legislation.
"No matter what safeguards are in place, evidence shows us that vulnerable people in our society are at risk when legislation permits the premature ending of life," he said.
In a statement the Catholic Church said the legislation was "fundamentally incompatible with the Catholic faith".
"There is simply no clear way of legislating for euthanasia and assisted dying that does not fundamentally threaten the lives of the vulnerable in our society," the church said.
"This is a serious concern for Tasmanians, as the state has the largest and fastest growing older population in Australia, and the highest prevalence of people living with disability of any state or territory."
The church said people's suffering and pain should be "relieved in a way consistent with the dignity and sanctity of human life".
"The Church supports the role of palliative care which is oriented to caring for, and accompanying, a dying person and his or her carers in the final phase of life, upholding that person's dignity and respecting his or her spiritual, physical, emotional and social needs," the statement said.
Both the Liberal and Labor parties will give members a conscience vote on the legislation.
If it is passed in the Legislative Council it will go to the House of Assembly where the last vote in 2017 was lost 16 votes to eight.
Independent Clark MHA Madeleine Ogilvie said she would "carefully consider" any bill.
"I have also made a commitment to ensure all law reform is well consulted," Ms Ogilvie said.
"I think it is important to note that should this issue come before the House of Assembly, it would likely be a conscience vote. In that circumstance my personal vote is one of 25 and the issue would be decided on the majority as it has been in the past."