Some 1,700 doctors and nurses have said they will refuse to take part in assisted suicides if the practice is legalised.

Baroness Meacher's Assisted Dying Bill is being debated by peers in the House of Lords on Friday.

It proposes changing the law so that the terminally ill with less than six months to live can request assistance in the form of lethal drugs to end their lives.

But doctors and nurses have expressed "great concern" over the proposals as they warn that any change to the law will leave the vulnerable at risk of "abuse", and undermine the patient-doctor relationship.

In a joint letter sent to Health Secretary Sajid Javid ahead of the debate, they warn parliamentarians not to overlook the enormity of the shift being proposed.

"As healthcare professionals, we have a legal duty of care for the safety and wellbeing of our patients. We write with great concern regarding the introduction of a Bill to legalise assisted suicide," they say.

The letter continues: "The shift from preserving life to taking life is enormous and should not be minimised ... The prohibition of killing is present in almost all civilised societies due to the immeasurable worth of every human life."

The letter goes on to say that it is "impossible" for any government to legislate against extension or expansion of the laws in future, something critics fear will happen because of trends in countries where assisted suicide is already legal, like Canada, Belgium and the Netherlands.

"Canada has clearly demonstrated that safeguards can be eroded in a matter of just five years," they warn.

"The prohibition of killing is the safeguard. The current law is the protection for the vulnerable.

"Any change would threaten society's ability to safeguard vulnerable patients from abuse, it would undermine the trust the public places in physicians, and it would send a clear message to our frail, elderly and disabled patients about the value that society places on them as people."

In the letter, organised by campaign group Our Duty of Care, they instead call for investment in quality palliative care for all at the end of life.

Spokesperson Dr Gillian Wright said the public were being "misled" about how assisted suicide laws would actually operate, and the risk of people feeling pressured to end their lives in this way simply to avoid being a burden on families, as evidenced by research in Oregon last year where this was given as the reason for over half of assisted suicides in the US state.

Dr Wright also pointed to evidence in Belgium showing that some assisted suicides have been carried out without any consent from the patient.

"Far from one person's decision affecting no one else, it affects us all. Some patients may never consider assisted suicide unless it is suggested to them," said Dr Wright.

"The cruel irony of this path is that legislation introduced with the good intention of enhancing patient choice will diminish the choices of the most vulnerable.

"We would not take patients' lives – even if they asked us to – but for the sake of us all, and for future generations, we ask that the law remains unchanged."

Dr David Randall, a medical registrar from London stated: "This letter emphasises just how much opposition there is within medicine to the legalisation of assisted suicide. The current law works well, protecting the vulnerable and allowing us to deliver to patients the kind of compassionate, individualised care to which we aspire.

"A change in the law would distort conversations and priorities around end-of-life care, and would threaten the world-leading hospices and palliative care services that we enjoy in this country.

"We call on politicians to keep the current law in place, and not to send to vulnerable patients the message that society no longer values their lives."

Their concerns were echoed by faith leaders in another significant intervention this week.

The Archbishop of Canterbury, Chief Rabbi and the head of the Roman Catholic Church in England said in a joint letter to peers that efforts to support the terminally ill should be focused on "assisted living" rather than assisted dying.

"All people of faith, and those of none, can share our concern that the common good is not served by policies or actions that would place very many vulnerable people in more vulnerable positions," they write.