Sir Graham Brady said: "The threshold of 15% of the parliamentary party seeking a vote of confidence in the leader of the Conservative Party has been exceeded".
Betfair spokeswoman Katie Baylis said: "While Theresa May is under vast pressure to retain the leadership ahead of tonight's vote, we are seeing a huge amount of backing from punters since the vote was announced this morning for her to prevail".
The news comes after months of criticism directed at Mrs May over draft Brexit deal and her handling of negotiations.
It is thought that, if the vote went ahead, May would not have had enough support for the agreement to be accepted, and she has reportedly cancelled the vote in anticipation of a certain defeat, according to American news company Bloomberg.
"However if she was to lose the vote, Dominic Raab looks the man to step into Number 10 and is the 9/2 favourite to take over, with Sajid Javid at 5/1, alongside Boris Johnson whose odds have slightly shortened this morning".
But she said she believed she could still get the deal through if she addressed their concerns.
"The votes will be counted immediately afterwards and an announcement will be made as soon as possible in the evening", he added.
Stephen Barclay also said Theresa May could stay in post if, as expected, MPs reject her Brexit plan.
What happens in the no confidence vote?
Brexit Secretary Steve Barclay voiced his support.
Tory MPs say it is unacceptable because it would result in new regulatory barriers between Northern Ireland and the rest of the United Kingdom and could continue indefinitely, because the United Kingdom would not be able to leave without the EU's approval. We need to back theresa-may and deliver the referendum result. The global development minister has reportedly been critical of May´s Brexit deal in private although she has remained outwardly loyal.
House of Commons Leader Andrea Leadsom said: "Vital to support Theresa May today".
She said: 'Like many people, there are parts of the current deal that I'm uncomfortable with so I'm keen to see if the PM can use the extra time to secure some improvements'.
"So there are plenty of options for the PM to talk to the European Union about that don't involve reopening the Withdrawal Agreement, but that would provide the legal text as a part of the Withdrawal Agreement, through perhaps an addendum".
He pulled his support from Johnson at the last minute - scuppering the former Mayor's bid, and laying out his own stall to be PM.
The 44-year-old was appointed Brexit minister in July, to replace David Davis who quit over May's deal, only to resign himself in November over the latest agreement struck with Brussels.
But Tory MP Nigel Evans said the prime minister had taken a "wise decision" as the outcome of the vote would have been a "humiliation" for her.
Theresa May has vowed to fight an effort to oust her as Conservative leader and Prime Minister "with everything I've got".
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