Three former Conservative prime ministers are against the assisted dying bill, it has been revealed.

Theresa May, Boris Johnson and Liz Truss have expressed their opposition to a change in the law, days before MPs vote on assisted dying proposals affecting patients in England and Wales for the first time in almost a decade.

May was expected to vote against Friday’s bill, the Daily Telegraph reported, noting her position had not changed since she voted against the legislation in 2015.

Johnson cannot vote because he is not a member of parliament, but he said he would not support the legislation as it stands. Rishi Sunak previously said he would back a change in the law over assisted dying, but it has been reported that it does not necessarily mean he will support the bill.

Truss told the Telegraph she was “completely opposed” to the bill saying: “It is wrong in principle: organs of the state like the NHS and the judicial system should be protecting lives, not ending them.

“No doubt, as we have seen in Canada, vulnerable people would be put under appalling pressure to end their lives early.

“The law would be ripe for being exploited by the unscrupulous. MPs should vote this terrible bill down and instead focus on improving health services.”

The former Labour prime minister Gordon Brown last week declared his opposition, saying the death of his newborn daughter in January 2002 convinced him of the “value and imperative of good end-of-life care”.

Writing for the Guardian, Brown said the assisted dying debate was moving too fast given the “profound ethical and practical issues” involved and called for a commission to devise a “fully funded 10-year strategy for improved and comprehensive palliative care”.

Tony Blair is yet to express his view on the issue, while Keir Starmer said he favoured a change in the law but also has not revealed his position.

However, the former Labour leader Neil Kinnock privately told colleagues he would support a change in the legislation and would vote for it in the Lords, the Guardian understands.

On Tuesday night, the mayor of London, Sadiq Khan, said he opposed the bill because of his fears around coercion and a feeling of guilt among terminally ill people.

Khan told the London Standard: “If I was a member of parliament, I’d be voting against the assisted dying bill. That’s not to say that those who feel strongly in favour of it are wrong.

“I think it’s right that the government has made this a free vote so MPs aren’t being whipped. I think it’s right that it’s a matter of conscience.

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“But I’ve got real concerns in relation to the lack of palliative care available to those who are terminally unwell. I’ve got concerns about the state of the NHS.

“I’ve got concerns about the state of social care provision.

“I am concerned not just about coercive control, I am concerned about some of the guilt those who are terminally ill may well have. For those reasons, if I had a vote, I’d be voting against.”

Munira Wilson, the Liberal Democrat MP who is part of a cross-party group of parliamentarians who backed an amendment that would “decline” to give a second reading to the bill if it is selected by the speaker on Friday, expressed her concern about the process of how the bill is to go through parliament.

The amendment, which could be considered on Friday, has also been backed by new Labour MPs Anna Dixon, Polly Billington, Josh Fenton-Glynn and Uma Kumaran, as well as the Conservative Ben Spencer.

Kim Leadbeater, the Labour MP who proposed the private member’s bill, told BBC Radio 4’s Today programme MPs had “absolutely” been given enough time to digest the details of the bill.

She added: “If this bill does pass the second reading, the government mechanisms will then start to kick in, basically, and we’ll start to look at what the implementation would look like. The debate will continue … and the advantage we’ve got … is that there are other countries around the world which have got a version of assisted dying, and we can learn from those.”