A theologian and retired surgeon has spoken out against Margo MacDonald's new proposal to legalise assisted suicide in Scotland.

Professor Donald MacDonald, a Fellow of the Royal College of Surgeons of Edinburgh and professor of practical theology, says that the current laws protect the most vulnerable and should not be modified.

A wheelchair user with multiple sclerosis himself, Professor MacDonald said that the qualifying conditions for assisted suicide outlined in the proposal are "so vaguely defined that that seem fairly elastic".

Margo MacDonald first attempted to pass a bill permitting euthanasia in Scotland through the End of Life Assistance Bill three years ago in 2010, but her efforts were blocked by strong opposition from MSPs in Parliament – it was defeated by 85 votes to 16 with two abstentions.

Ms MacDonald has now relaunched her proposal in the hope that greater awareness of the issue among the public will mean more support for her campaign. She has claimed she is "pretty certain" that support has increased among MSPs since her last failed attempt.

The Independent MSP, who suffers from Parkinson's disease, outlined her Assisted Suicide Bill in Edinburgh several weeks ago. If passed, it will allow people with progressive degenerative conditions or terminal illness to seek a doctor's help in dying.

Ms Macdonald has also indicated future plans, asserting that if the bill is passed and able to operate effectively for a number of years, there may be opportunity for further developments in the law that would offer hope "to other categories of people seeking assistance to die".

Professor MacDonald previously spoke against Margo MacDonald's attempt to legalise euthanasia in Scottish Parliament in 2010.

He said her latest bill would mark "a hideous departure from the Hippocratic and Christian tradition" of the medical profession.

He has criticised several measures in the bill, including one which suggests the provision of a licensed facilitator or "friend at the end".

"[It is] is a bizarre innovation, and the suggestion that all this should be videoed is sickening and voyeuristic.

"What happens if the person vomits the drug and does not die? What if the person goes into a coma and refuses to die?" he has questioned.

"The current laws exist to protect the vulnerable and should not be changed," he concludes.