Speaking outside 10 Downing Street this morning, Mrs May said: "I will contest that vote with everything I have got".
"The new leader would not have time to negotiate a new withdrawal agreement and get the legislation through parliament by the 29th of March".
The threshold of 48 letters calling for a leadership vote from Tory MPs was passed, triggering a vote of no confidence. It comes days after May postponed a vote to approve the divorce deal to avoid all-but-certain defeat in the House.
Former Environment Secretary Owen Paterson accused May of acting like a "supplicant" in dealings with the EU.
Foreign Secretary Jeremy Hunt condemned the challenge as "the last thing the country needs".
May said changing Conservative leader would "put our country's future at risk and create uncertainty when we can least afford it".
MP Peter Bone said it was clear she had failed to win a majority of backbenchers, pointing out that ministers and the "payroll vote" amounted to more than 150 MPs.
The Conservatives had to build a "country that works for everyone" and deliver "the Brexit people voted for". If she loses, she must resign and is barred from standing in the leadership election that follows.
From the standpoint of domestic United Kingdom politics, the agreement satisfies few.
A challenge was triggered after 15 per cent of the Conservative members of parliament (MPs) wrote letters demanding a confidence vote to the chairman of the party's "1922 Committee", which represents those lawmakers who have no government jobs.
May could also be ousted by MPs from within her own Conservative Party and British media are full of speculation on potential contenders.
The vote will take place until 8pm, with the result announced soon afterwards.
If she had lost the confidence vote Mrs May would have been forced to stand down as Conservative Party leader, and then as prime minister.
But the BBC's Laura Kuenssberg said the result was "not at all comfortable" for the prime minister and a "real blow" to her authority.
If she wins the vote, however, her Tory foes would be unable to launch another leadership challenge against her for a year.
House of Commons Leader Andrea Leadsom said: "Vital to support Theresa May today". If all elected lawmakers cast their ballots, that would now mean 158 votes. But allies said May would stay in post even if she secured a wafer-thin win.
She added: "None of that would be in the national interest".
The no confidence vote itself has once again exposed the division within the Conservative Party over the European Union, but a leadership contest would have amplified it.
Home Secretary Sajid Javid, who is known to have leadership aspirations, and his predecessor Amber Rudd, have also offered their support to the Prime Minister.
Ahead of the vote, cabinet colleagues rallied to May's support.
The PM is then scheduled to attend a summit of European leaders on Thursday at which she is likely to press for changes to elements of her Brexit deal to try to get the support of Parliament.
"Brexit was never going to be easy but she is the best person to make sure we actually leave the European Union on March 29".
George Freeman, a former May adviser who had urged her to promise to step down next year, said he would vote for her, as did the Harlow MP, Robert Halfon, who said the prime minister had promised to make social justice her first priority.
For all Xherdan Shaqiri's prompting, Liverpool lacked penetration and any element of surprise without their usual front three. Using Daniel was clear; Div after the last game makes sense, using the momentum, trying to use it.
The estimate by Forbes magazine found that Ryan of Ryan ToysReview pipped Jake Paul by $US500,000 for the 12 months to June. The video featured Ryan opening and reviewing a box containing more than 100 toys from Pixar's " Cars " series.
Bush from another member of that club. "As President, he set the stage for decades of prosperity that have followed". Bush took his final flight, reuniting with his wife of 73 years, Barbara and his 3-year-old daughter, Robin".
She said she got to what she thought was her apartment - Jean's was directly above hers - and found the door ajar. Jean's family filed a lawsuit against the City of Dallas and its police department in late October.
Trump weathered the similar criticism of going soft on Mr. "I want the Ukrainian soldiers released", she said. He stopped short of condemning Russian Federation , instead saying: "I don't like that aggression".
Espy and Hyde-Smith, who would be the state's first elected female senator, are vying for the seat that was long held by Sen. The discovery comes just before Tuesday's runoff election between Democrat Mike Espy , who is black, and Republican Sen.
Mrs May had a meeting with the prince on the first day of the G20 summit of world leaders in Buenos Aires , Argentina. Saudi Arabia denies that bin Salman ordered the killing, but US intelligence agencies have concluded the opposite.
The plane's cockpit voice recorder, which would provide more information about what happened in the cockpit , has yet to be found. National Transportation Safety Board as technical advisers to support the ongoing investigation" by the Indonesian authorities.
Tariffs have proven taxing on many companies, especially those with global supply chains and suppliers in China. US business leaders welcomed the news of a delay of an increase in the tariff rate between the USA and China.
The custom-painted unit incorporates the colors of the Air Force One used during Bush's years as president, from 1989 to 1992. Bush's body is scheduled to lie in state at the US Capitol from 5 p.m. on Monday until 10 a.m. on Wednesday.
Mordaunt is one of the last remaining pro-Brexit members of May's cabinet, where she serves as global development minister. MP Graham Brady says the threshold of 48 letters from lawmakers needed to trigger a leadership vote has been reached.
The aircraft lost communications for almost an hour, forcing it to make an emergency landing in Germany's Cologne-Bonn airport. He also said that the back of the Buenos Aires government delegation will take another plane, the air force of Germany.
WSMV reported that the replacement comes after Rock made derogatory comments about Joy Behar on an episode of Fox & Friends. On April 22, Shaw tackled and unarmed a shooter at the Waffle House in Antioch, Tennessee, a southern suburb of Nashville.
The senator added, "I don't think the administration won any votes [to oppose the Senate resolution] in that troubled briefing". Pompeo declined twice to say why Haspel wasn't there, responding with a smile, "I was asked to be here, and here I am".
We kept him on because we thought he had potential to be a big steer but never imagined he'd grow to be this huge, ' he said. An animal dubbed Australia's biggest steer has been saved from the slaughterhouse - because he's just too huge to deal with.
The movement has no clear leaders, and past protests have attracted extremists who hurled projectiles at police. Mr Macron will speak from the presidential Elysee Palace at 8pm (1900 GMT), an Elysee official said.
The president's next chief of staff will be his third in under two years. -Treasury Secretary Steve Mnuchin. Ayers, Axios reported , will run Trump's outside group, America First, after leaving the administration.
The woman died about a month later. "Repeat CT imaging demonstrated further hemorrhage into the original resection cavity". After she died, doctors determined the specific infection had been caused by an amoeba called Balamuthia mandrillaris .
Dunford originally was nominated for the Joint Chiefs position by former President Barack Obama, serving an initial two-year term. The chairman is the highest-ranking officer in the US military and is the top military adviser to the president.
Although the flagship series is ending, HBO has multiple projects set in the Game of Thrones universe now in development. We see the Stone Table, that great map of Westeros located in the castle at Dragonstone, begin to freeze from the north.
It was earlier this week when Mika Singh took to his social media handle and shared a video from Dubai saying, "Hey Dubai". Mika was reportedly picked up from Bur Dubai, a historic and commercial district with many hotels, around 3 am on Thursday.
Trump is a traitor to our country ". "Rudy didn't separate g-20 from.in so ya boy bought the domain", that person said. After discovering the gaffe, Giuliani tweeted on Tuesday that he didn't mistype the tweet.
In theory, the most severe penalty is expulsion from the House, although the prospects of that happening would appear remote. Labour could also try to cobble together a coalition with other parties, but now this looks impossible.