"The new leader wouldn't have time to renegotiate", she added, "so one of their first acts would have be extending or rescinding Article 50".
Conservative MPs will vote from 18:00 GMT to 20:00 GMT.
The vote will be held tonight, with the result expected at around 9pm.
Immediate statements of loyalty for the Prime Minister were issued by Cabinet ministers, including several who have been named as potential contenders to be her successor.
"A leadership election risks handing control of the Brexit negotiation to opposition MPs in Parliament", she said. If all elected lawmakers cast their ballots, that would now mean 158 votes.
But it is a secret ballot so the final outcome is hard to predict.
May will now need the backing of at least 158 Conservatives MPs to see off the challenge to keep her position safe for another 12 months.
The PM will hope to win enough concessions to persuade rebel MPs to vote her deal through.
"I think the idea of getting rid of the Prime Minister with all that that would entail, with a leadership contest, would be just hugely irresponsible", said Tory MP Anna Soubry who has opposed Ms.
"She said that in her heart she would like to fight the 2022 election, but that she recognised the party did not want her to, and therefore it was not her intention to", he said.
Solicitor general Robert Buckland, also speaking outside the meeting, said: "the party has to be prepared to give her the space to go back to the EU".
But she is now expected to travel to a summit in Brussels on Thursday to continue trying to persuade European Union leaders to change the deal - they have previously said it cannot be renegotiated.
"I have devoted myself unsparingly to these tasks ever since I became prime minister and I stand ready to finish the job".
Trump says he may intervene in Huawei case
Huawei, the biggest global supplier of network gear for phone and internet companies, is the target of USA security concerns. He said the ICG was not registered in China as a non-government organisation and Kovrig could have broken Chinese law.
These two candidates are then put to a postal ballot of the wider Conservative Party membership.
The last time such a ballot took place was in 2005, when David Cameron and David Davis spent over a month competing for Conservative members' support, before Cameron was announced the victor.
"She believes it is her duty to serve as long as the party wants her to". One trenchant critic, the North East Derbyshire MP, Lee Rowley, told her: "Stamina is not a strategy".
But he said the DUP would not support a no-confidence motion in Parliament at this stage.
What does it mean for Brexit?
That arithmetic can only be changed by a general election, an election that many Tory MPs in marginal seats simply can't countenance because of the real threat of Jeremy Corbyn running yet another poll-defying election campaign that could make him Prime Minister.
Many supporters of Brexit say May's deal, a compromise that retains close economic ties with the European Union, fails to deliver on the clean break with the bloc that they want. They say warnings of economic doom are exaggerated.
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.
But other MPs said replacing Ms.
Many demonstrators continue to protest against Brexit outside the Houses of Parliament, with some calling for a new "peoples' vote" or for the United Kingdom to ditch Brexit altogether.
The party restored voting rights to two lawmakers who had been suspended for misdeeds, bringing the total to 317 from 315 earlier in the day.
I realise that the party would like a different leader to take them into that election.
She said: "The Conservatives have, yet again, put their own self-interest and petty squabbling above the national interest".