A monumental decision for MPs that has been looming on the horizon is now approaching at speed.

The Assisted Dying Bill, which would give some terminally ill adults the right to end their lives, is back in the Commons for votes on amendments, with the final vote likely to come next week.

Sky News has learned that 20 MPs have changed their position on the legislation since it was first voted on.

There are not many issues like this – literally a matter of life and death – that require MPs to search their consciences and make a personal decision with profound and irreversible consequences.

When the Commons first voted on the legislation back in November it passed with a 55 majority.

That may seem reasonably comfortable but delving into the numbers reveals that it is flimsier than it first appears.

Firstly, it would take just 28 MPs to switch from for to against to bring the bill down.

And some of that initial support is quite soft with a significant number voting in favour simply to move the bill to the next stage, have the debate, raise the profile of the issue and make progress on the detail.

Now that the final vote is drawing near, they may decide it’s gone far enough.

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There are also the abstainers who could go either way if they do choose to vote at third reading.

All this makes the outcome unpredictable, but Sky News has been speaking to MPs to get a sense of where the parliamentary arithmetic is headed.

Ahead of the latest round of votes, 20 have confirmed to Sky that their position has changed and although there is movement in both directions it shows that the momentum is mostly one way.

Taking this snapshot of MPs, the first time round nine voted for, nine abstained and three voted against.

Based on how they say they will vote at the third reading, the numbers are very different – with just four in favour, one abstaining and 15 against.

Among those are two ministers, Sir Chris Bryant and Ellie Reeves who are swimming against the tide by declaring that they will now vote in favour.

Sir Chris told Sky News: “I abstained on the first time round, I decided I wasn’t going to vote because I wanted to hear the debate. I have listened to a lot of the debate…

“I also have heard the cries of people who are absolutely miserable, and that’s why I will be voting for the bill.”

Of the many more MPs who have withdrawn their support, Conservative George Freeman is one of a handful who have spoken out. He told Sky: “I want to see a law change, I think Kim [Leadbeater]’s done us a favour…

“I think government needs to come back with a properly thought through, properly consulted on, possibly by royal commission. This is a big change, so I want to see law change, but I’m no longer going to vote for this bill.”

And skewing our figures slightly is Labour MP Emma Hardy, who voted both for and against at second reading but has now settled on against.

Just that small number of switchers would still see the legislation over the line, but with a reduced majority of 38, and many believe the vote will be much closer.

The main issues that are concerning undecided MPs are, firstly, protecting patients from being forced or coerced into ending their own lives.

At the beginning of the parliamentary process it was suggested that a high court judge would sign off every assisted death.

That has now been changed to a panel of experts, and some MPs are worried that this represents a watering down of safeguards. Kim Leadbeater, who is behind the law change, says this will actually make it safer.

Others point to the way the legislation has been brought to parliament, as a private member’s bill, which some argue reduces its robustness. They would like to see it return with the full weight of government behind it.

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And finally, changes to the timeline have raised a lot of eyebrows with the maximum implementation period extended to four years.That has raised concerns that the bill could be passed in this parliament but not delivered, and would become a divisive issue at the next general election.

Despite all this there is still a huge amount of support and all eyes will be on the House of Commons as more debate and votes on amendments get underway, giving us the best sense yet of whether this once in a generation legislation still has a chance.