Brown, a longstanding critic of assisted dying, told BBC Radio 4’s Sunday programme: “An assisted dying law, however well intended, would alter society’s attitude towards elderly, seriously ill and disabled people, even if only subliminally, and I also fear the caring professions would lose something irreplaceable – their position as exclusively caregivers.”
Brown, Johnson and Truss will not get a vote on the issue as they are no longer MPs.
However Lord Cameron, appointed a peer by Rishi Sunak to serve as foreign secretary, pledged to vote for the bill if it reached the House of Lords.
The last time there was a vote on legalising assisted dying in the House of Commons in 2015, he did not record a vote.
Sources close to Baroness May, who also sits in the Lords, said her views had not changed since she voted against legalising assisted dying in 2015.
MPs will get their first opportunity to vote on the bill proposed by Labour MP Kim Leadbeater on Friday.
Currently slightly more MPs have publicly said they will support it but more than half have not revealed which way they plan to vote, making the result hard to predict.
The Terminally Ill Adults (End of Life) Bill would allow terminally ill people expected to die within six months to seek help to end their life if two doctors and a High Court judge verified they were eligible and had made their decision voluntarily.
Leadbeater said the “status quo is not fit for purpose” and her proposals could prevent “very harrowing, very distressing deaths”.
Current laws in the UK prevent people from asking for medical help to die.
The bill would require those who apply for assisted dying to:
Be over the age of 18, a resident in England and Wales and registered with a GP for at least 12 months
Have the mental capacity to make a choice about ending their life
Express a “clear, settled and informed” wish, free from coercion or pressure, at every stage of the process.
Writing in The Times, Lord Cameron said: “Many of these safeguards will be familiar from previous proposals.
“But this new Bill protects the vulnerable still further, including by making coercion a criminal offence.”
He added: “Will this law lead to a meaningful reduction in human suffering? I find it very hard to argue that the answer to this question is anything other than ‘yes’.”