The victory ensures that no further leadership challenge can be mounted for 12 months under party rules, and Mrs May will be the UK's leader as it navigates its way through to Brexit, which is due in March 2019.
While the backstop would mean that no hard border emerged on the island of Ireland, it would also mean the United Kingdom stayed in the EU's customs territory with no obvious exit strategy and would require checks on goods between Northern Ireland and Great Britain, something unpalatable to many Conservative MPs.
Wantage MP Ed Vaizey has urged fellow Tory MPs to back Prime Minister Theresa May in tonight's no confidence vote. But May gave nothing away when it came to the Deal - the thing that has been the trigger for the No Confidence vote itself. "Just a handful of MPs can make a huge difference for us", the Labour source claims.
The DUP has consistently stated it would not engage in any moves that may hasten the prospects of a Corbyn-led government, but removing their support for May could lead to a snap general election.
Fellow seasoned Brexiteer Owen Paterson, who was said to have sparked this vote, shared his letter of no confidence in The Telegraph Tuesday night, writing that "the Prime Minister's proposed "deal" is so bad that it can not be considered anything other than a betrayal of clear manifesto promises".
May will address EU leaders on Thursday night at the European Council summit in Brussels as she seeks to obtain further changes to the Irish backstop, created to avoid a return to a hard border on the island of Ireland, but which has proved bitterly controversial among Conservative MPs.
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Legislators began casting their secret ballots in a wood-panelled room in Parliament after Mrs May's Conservative opponents, who have been circling for weeks, finally got the numbers they needed to spark a vote.
She pledged to seek "legal and political assurances" on the Brexit backstop to allay MPs' concerns about her Withdrawal Agreement when she attends a European Council summit in Brussels on Thursday. This is the vote that was originally scheduled for Tuesday night, but which the prime minister pulled from the schedule after recognising that MPs would reject the deal if they considered it then.
In a trip to Japan later in the year she insisted she would lead the Tories into the next election.
Appearing at a meeting of the backbench 1922 Committee to make her case before MPs cast their votes, Mrs May received an enthusiastic welcome with backers banging their desks in support.
And this comes after May had said this was the "only deal" and said it could not be changed. And we already know that this was going to be nearly impossible.
Theresa May has just days to persuade her European partners to agree changes to the deal.
It's becoming increasingly likely that the United Kingdom will leave on March 29 with no deal in place, a so-called hard Brexit that critics claim will lead to economic chaos.
"My focus now is on ensuring that I can get those assurances that we need to get this deal over the line, because I genuinely believe it's in the best interests of both sides - the United Kingdom and the European Union - to get the deal over the line, to agree a deal".
"A change of leadership in the Conservative Party now will put our country's future at risk", May said in a defiant statement outside 10 Downing St before the vote. Her lack of overall majority in the House of Commons continues to be her main domestic weakness.
To the Democratic Unionist Party, which props up Mrs May's Government, this undermines the Union by creating a regulatory barrier for goods crossing the Irish Sea.
One lawmaker who attended, James Cleverly, said May "made it very clear that there is a job of work to be done (on Brexit) and this is a delay and a distraction".