UK Prime Minister Theresa May will face a vote of confidence in her leadership later on Wednesday.
In a statement in Downing Street, Mrs May said: “I will contest that vote with everything I have got”.
She said a new prime minister would have to scrap or extend Article 50, the mechanism taking Britain out of the EU on 29 March, “delaying or even stopping Brexit”.
Conservative MPs will vote between 18:00 GMT and 20:00 GMT.
Mrs May said changing Conservative leader would “put our country’s future at risk and create uncertainty when we can least afford it”.
The challenge to Mrs May’s position comes after the required 48 letters calling for a contest were delivered.
Mrs May, who has been prime minister since shortly after the UK voted to leave the European Union in 2016, has faced criticism in her party for the Brexit plan she has negotiated.
It is not yet known how quickly the result of the secret ballot will be announced, but Mrs May needs to get a majority in her favour to win it.
If that happens she cannot be challenged for at least another year.
If Mrs May does not win the vote there would then be a Conservative leadership contest in which she could not stand.
There is also an option that even if Mrs May won – but not overwhelmingly – she may decide to stand down as party leader
Because the Conservative Party is the largest party in the House of Commons, whoever is a leader of the party would be expected to be prime minister.
If Mrs May is ousted as Conservative leader later she would be expected to stay on as a caretaker prime minister until a new leader is selected by the party, a process that could take six weeks.
If there are multiple candidates, Conservative MPs hold a series of votes to choose two to go forward to a vote of party members.
Sir Graham Brady, chairman of backbench Tories, who is overseeing the vote, said: “We are looking at the leadership of the party and clearly the prime minister remains until there is a successor.”
Sir Graham said he had told the prime minister on Tuesday evening that she would face a confidence vote. She had been “businesslike” and “was very keen that matters be resolved as quickly as was reasonably possible”, added Sir Graham.
Justice Secretary David Gauke told BBC Radio 4’s Today programme he would not expect a new leader to be in place until late January or February – meaning they would have to ask the EU for more time to negotiate Brexit.
“If she loses tonight whoever is prime minister will have to delay Article 50. I cannot see how we can possibly leave on 29 March.”
Mr Gauke said he was “disappointed” that the confidence letters had gone in, but said: “I hope that the prime minister will win tonight and win well.”
The BBC’s Political Editor Laura Kuenssberg said a delay to Brexit was “a vital part of the case the prime minister’s supporters will be making” as she faces the confidence vote.
Ministers have been tweeting their support for the prime minister – but Laura Kuenssberg said this was not necessarily a sign that they would back her in the vote, which is a secret ballot.
Labour’s Shadow International Trade Secretary Barry Gardiner said the Conservative Party was “putting the resolution of their own divisions ahead of the interests of the country”.
Mrs May was due to meet Irish premier Leo Varadkar in Dublin later on Wednesday, as part of her efforts to get changes to her EU withdrawal agreement to get it through the UK Parliament.
But she said she had cancelled that meeting to fight for her leadership.
Mrs May held talks with other EU leaders on Tuesday, who say the deal cannot be renegotiated.