UK Prime Minister, Theresa May, will face a vote of confidence in her leadership later on Wednesday.
Speaking outside 10 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 from 18:00 GMT to 20:00 GMT.
A result is expected an hour or so later.
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.
Mrs May said changing Conservative leader would “put our country’s future at risk and create uncertainty when we can least afford it”.
“A leadership election would not change the fundamentals of the negotiation or the Parliamentary arithmetic.
“Weeks spent tearing ourselves apart will only create more division just as we should be standing together to serve our country. None of that would be in the national interest,” she said in her statement.
She said she was making progress in her talks with EU leaders and vowed to “deliver on the referendum vote and seize the opportunities that lie ahead.”
The Conservatives had to build a “country that works for everyone” and deliver “the Brexit people voted for”, she said.
“I have devoted myself unsparingly to these tasks ever since I became prime minister and I stand ready to finish the job.”
The challenge to Mrs May’s position comes after the required 48 letters calling for a contest were delivered.
One of those who submitted a letter, former minister Owen Paterson, said: “She is sadly determined to stick to her deal which is not going to get through the House of Commons, and worse, if it did get through the House of Commons, she would lose the support of the DUP, which triggers a general election.”
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 thought the result of the secret ballot could be announced as soon as 21:00 GMT – 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 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.—BBC
Theresa May, UK Prime Minister