Had a majority voted against her, May would have been forced to resign as party leader and would have been expected to stand down as prime minister.
May will need the support of more than 50 percent of the 315 Conservative MPs to cling to power, or 158 in total. But her authority was seriously undermined by the botched early general election she called a year ago which, instead of consolidating the Conservatives' hold on power, ended up with Mrs May losing her overall parliamentary majority.
With less than four months left until the United Kingdom is due to leave the European Union on March 29, May's Brexit deal is floundering, opening up prospects that run the gamut from a disorderly no-deal divorce to calling Brexit off.
The threat to May has been building as pro-Brexit Conservative lawmakers grew increasingly frustrated with the prime minister's handling of Brexit.
These two candidates are then put to a postal ballot of the wider Conservative Party membership.
May put up a courageous front at the Prime Minister's Questions in the House of Commons at noon, taking on her critics and informing them that talks were on to improve the controversial withdrawal agreement, particularly on the backstop on Northern Ireland. Instead, she spent the day shuttling through European capitals, seeking changes to the deal to try to win over skeptical legislators before Britain leaves the EU on March 29.
The final pair then go to a postal ballot of all party members, with the position of leader - and prime minister - going to the victor.
If May loses, her successor would be chosen from existing Conservative MPs and would automatically become prime minister.
But a rising number of backbench lawmakers, along with three of the four living former prime ministers, say the only way out of the impasse may be a new referendum with an option to stay. If she loses the vote, the 1922 Committee would set out a timetable for a contest.
They echoed the prime minister's message that airing their divisions in public would undermine voters' trust in the Tory party - and it would be hard for any new leader to change the course of Brexit before the March 29 date on which Britain is due to leave the EU.
In her statement delivered early on Wednesday morning (UK time) Mrs May said: "A leadership election would not change the fundamentals of the negotiation or the Parliamentary arithmetic".
According to two sources, Merkel told her own German conservative parliamentary group on Tuesday that there would be no further negotiations on Brexit but efforts were being made to give Britain reassurances. A confrontation over her leadership became inevitable early this week, when the premier suddenly chose to stop the ratification of the Brexit deal in Parliament, largely because she knew the deal would be decisively rejected by a large majority of MPs.
The vote of no-confidence in her leadership comes amid the widening chaos over the manner of the U.K.'s exit from the European Union.
Graham Brady says the threshold of 48 letters from lawmakers needed to trigger a leadership vote has been reached.
The delay also prompted outrage among lawmakers, with one leading Conservative rebel, Brexit-supporter Mark Francois, accusing the government of having "hidden in the toilets".
Dominic Raab, another former Brexit Secretary.
Environment secretary Michael Gove added, "I think the PM will win tonight and she will win handsomely".
Anti-Brexit campaigners wave the Union Jack and European Union flags outside the Houses of Parliament on Wednesday.
Still, a victory by the narrowest of margins is unlikely to strengthen her authority.