Mahmood joined Health Secretary Wes Streeting and Education Secretary Bridget Phillipson who have already said that they would vote against the bill.
Care Minister Stephen Kinnock expressed his support for the bill, while Prime Minister Sir Keir Starmer backed assisted dying proposals the last time they were debated by MPs in 2015. This time round he has said he will not pressurise Labour MPs either way.
Campaigners supporting the bill, including broadcaster Dame Esther Rantzen, argue terminally ill people should get a choice over how they die to avoid unnecessary suffering.
Dame Esther, who revealed last year she had joined Dignitas after a terminal cancer diagnosis, described the new bill as “wonderful”.
In her letter, Mahmood said the most significant risk of introducing assisted dying is “the pressure the elderly, vulnerable, sick or disabled may place upon themselves” to end their lives sooner.
She said she was “profoundly concerned” by the legislation, not just for religious reasons but because it could create a “slippery slope towards death on demand” and argued the government must “protect and preserve life, not take it away”.
Mahmood added that scandals like Hillsborough, infected blood and the Post Office Horizon “reminded us that the state and those acting on its behalf are not always benign”.
Speaking to Sky News, Leadbeater said she had no doubts “whatsoever” her proposed bill was the right thing to do.
Current laws are “failing” causing families to lose loved ones in “harrowing circumstances” and leaving people to endure “painful deaths”, she said.
“By creating a legal framework, we will improve the situation,” she added.