Professor John Wyatt has warned that many Christian doctors will not in conscience be able to practise in the NHS if assisted suicide becomes legal.
The President of the Christian Medical Fellowship (CMF) was speaking at a meeting of Christians on the Left ahead of a House of Lords debate on Baroness Meacher's Assisted Dying Bill on 22 October.
He told the online meeting that if "we do change the law, we are going to affect hundreds of thousands of people who die every year".
Out of the half million who die in the UK each year, the majority are terminally ill and would qualify for "receiving lethal drugs" if Baroness Meacher's Bill becomes law, he said.
He warned that because of other medical laws, NHS doctors would be required to inform large numbers of terminally ill patients "desperately worried about being a burden" that "they have the right to kill themselves".
"I am deeply concerned that a lot of Christian doctors will come to the conclusion: 'I simply can't practise in the NHS under these circumstances. It is so unacceptable for me to be having to be telling this succession of people something which I think is quite wrong, offering them the possibility of killing themselves'," he said.
Professor Wyatt highlighted the "very strong public support for a change in the law" shown in a recent YouGov poll revealing that 73 per cent are in favour of Baroness Meacher's Bill.
Support was evenly spread among Conservative and Labour voters. But among MPs, 35 per cent were in favour, 35 per cent against, and the rest undecided, he said.
The Christians on the Left meeting was chaired by former Labour MP Andy Reed.
Also speaking was CMF chief executive, Dr Mark Pickering, who highlighted "the inevitability of incremental extension" in the numbers of people becoming eligible for assisted suicide.
In many countries where euthanasia has been legalised "the safeguards that were put in place to begin with" soon become seen as "barriers to patient access and the campaigns come very quickly to push them back and to knock them down", he said.
"We have to think: why would the UK be any different?" he added.
Wyatt, who is also Emeritus Professor of Neonatal Paediatrics at University College London, has written a Parliamentary Briefing for the Lords in which he presents his views of the flaws in Baroness Meacher's Bill. This is due to be circulated to peers next week.