The challenge to the Prime Minister was triggered after at least 48 MPs wrote letters to the 1922 Committee calling for a leadership ballot.
There is no confirmation but sources, including a cabinet minister, have said they believe 48 letters have been sent.
Sir Graham Brady, the chairman of the 1922 committee has confirmed that a ballot will be held this evening. They say warnings of economic doom are exaggerated.
Hitting out at Theresa May for a lack of clarity on when the withdrawal deal would be put before the House, Labour leader Jeremy Corbyn said: "The Prime Minister's appalling behaviour needs to be held to account".
Some Tories who voted Remain in the referendum are unhappy too.
This is usually done by MPs who have lost confidence in the leader of their party, but can also be triggered by the Opposition.
What Happens During A Confidence Vote? Otherwise, the Queen would have to dissolve Parliament.
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He said the best Britain's prime minister could hope for was "further clarity and further interpretations without reopening the withdrawal agreement" when she meets European Union leaders in Brussels this week.
The vote could take place next week or even be delayed until early January, although this would allow less time for the ensuing Brexit legislation to be passed through parliament before 29 March.
Image copyright Getty Images Image caption Theresa May met German Chancellor Angela Merkel in Berlin on Monday What have Conservative backbenchers said?
The magic number to win was 159 votes, however, the margin of victory is unlikely to silence her critics and deep divisions over the terms of the UK's departure from the European Union remain.
But some ministers expressed support for her with Home Secretary Sajid Javid saying a leadership contest was the last thing Britain needed.
Predictions as to how many Tory lawmakers will support May in the vote now vary between around 110 and 170, with the Prime Minister needing 159 to survive. "There is no room whatsoever for renegotiation", European Commission head Jean-Claude Juncker said in an address to the European Parliament in Strasbourg.
The field was narrowed to Mrs May and then-junior minister Andrea Leadsom, but she pulled out before members voted, leaving Mrs May to become leader unopposed. She will remain leader, and prime minister, until the successor is picked.
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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.
The ballot has opened for MPs to start voting on Theresa May's leadership.
In the last leadership contest in 2016, members never got to vote because all the candidates dropped out but one: May. Other political editors said the threshold had not been reached.
Mrs May has been prime minister since 2016, taking up the position shortly after the United Kingdom voted to leave the European Union.
Mrs May is now immune from a leadership challenge for a year.
Bone said: "She said in 2017 she would only lead the Conservative party if she had the support of the Parliamentary party".
Brady said that the party aimed to finish the first stage of any leadership contest - in which lawmakers vote to whittle down the field of contenders to two - before Parliament breaks for Christmas on December 20.
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Conservative MPs will vote from 18:00 GMT to 20:00 GMT.