The outcome of the assisted dying vote is on a knife edge.

No one in Westminster truly knows which way it will go.

But why is it so?

A vote on assisted dying was supposed to be one of the easiest reforms for Keir Starmer’s government.

The last time MPs voted on it in 2015, they overwhelmingly rejected it.

However, public opinion has changed dramatically in the past seven years.

New figures from pollster YouGov indicate that more than seven in 10 people agree with the principle that assisted dying should be legal.

And a recent Daily Express poll showed eight out 10 readers back a change in the law.

To many, a law allowing terminally ill patients to choose to die would be a progressive and historically significant change.

So why is the vote on November 29 going to be so close?

The problem ultimately lies with Sir Keir.

Conscious of the complexity of the issue, the Prime Minister didn’t want to have to order his MPs to vote for assisted dying.

His strategy instead was to use a private members’ bill, brought by the Labour backbencher Kim Leadbeater.

But the ploy didn’t work because a strong opposition has emerged, and there is no guarantee that the bill will pass on Friday.

The magic number for winning the vote is around 320, less if some MPs abstain.

As it stands the Express understands that at least 127 MPs will vote in favour.

About 100 are thought to be wavering either way.

Labour and the Lib Dems have more MPs publicly pro, while the Tories have more publicly against.

To make matters worse for Starmer, the bill’s main critic is not from the opposition benches but his own side, Health Secretary, Wes Streeting.

Streeting has not only said that he will be voting against but that he is doing so because he fears the bill could harm existing health services.

His very public interventions in recent weeks have provoked a furious backlash, including from within Labour.

The Health Secretary initially said that palliative care on the NHS was not good enough to guarantee people a fair choice.

He also raised concerns about the potential drain on NHS resources and ordered officials to assess the likely cost.

And has highlighted fears that some people could feel pressured into ending their own lives to avoid being a burden on their loved ones – something he described as a “chilling slippery slope”.

Labour’s former deputy leader Harriet Harman accused Streeting of “crossing the line” by suggesting that his department was opposed to the move.

Leadbeater described his comments as “quite disappointing and quite upsetting”.

The Prime Minister is reported to have personally rebuked Streeting in private for not staying out of the debate.

Starmer’s views on the matter are pretty clear as he has a long history of supporting the decriminalisation of assisted suicide.

Last year he said there were “grounds for changing the law” on the issue.

In March this year he promised campaigner Dame Esther Rantzen that he would make time for a free vote if Labour won the election, saying he was “personally in favour of changing the law”.

But some have accused him of “backing away” from the debate while Streeting is taking the lead.

One Labour source said the PM’s “lack of leadership” had “created a vacuum” for those who want to stop the change from happening.

“This could end up quite embarrassing for the Prime Minister, that’s not what he needs after the last few months,” they said.

Others have accused Sir Keir of lacking a backbone on the issue.

“Like many things he just likes to see what way the wind is blowing and goes with it. Streeting has been allowed to publicly speak his mind and influence the undecideds.”

Divisions in the Cabinet over the issue have also deepened – with Education Secretary Bridget Phillipson suggesting she would vote against a law change.

Meanwhile Transport Secretary Louise Haigh said she intended to vote for a change in the law.

Justice Secretary Shabana Mahmood said last month she will not support the Bill, while Culture Secretary Lisa Nandy has stated she will vote yes.

Chancellor Rachel Reeves last week declined to give her view, saying she will be “looking at all the evidence ahead of the vote in Parliament”.

Home Secretary Yvette Cooper has also suggested she could back it.

Kit Malthouse is one of the most senior Tory MPs to publicly state he is in favour off assisted dying.

Conservative leader Kemi Badenoch has said she is supportive in principle but has reservations about whether the change can be implemented within the current system.

Many of those opposed to a law change have voiced concern about the potential for coercion and mission creep, and say the legislation has been rushed.

But proponents argue existing legislation lacks proper safeguards, fails to respect patient autonomy and discriminates financially between those who can afford to travel abroad to end their own lives within the law and those who cannot.

Ms Leadbeater has described her Bill as the most “robust” in the world, with “three layers of scrutiny” in the form of a sign-off by two doctors and a High Court judge.

It would also make coercion an offence with a possible punishment of 14 years in jail.

The Terminally Ill Adults (End of Life) Bill is backed by the Daily Express Give Us Our Last Rights crusade and Dame Esther.

The campaigner, 84, has registered with Dignitas after being diagnosed with stage four lung cancer last year.

The Bill, which covers England and Wales, states only terminally ill adults with under six months left to live and a settled wish to die would be eligible.

Another reason for the tension is the genuine philosophical debate.

Campaigners on both sides of the assisted dying debate are fiercely lobbying MPs and hosting events in Westminster.

Humanists UK this week hit out at Christian Action, Research and Education for encouraging people to write to their MPs using an online template that makes no mention of religion.

It comes after an investigation found that some opposition groups presenting themselves as grassroots campaigns were financed by conservative Christian pressure organisations.

If successful, further debates and votes would be needed before the bill becomes law.