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.
By BBC Political Editor Laura Kuenssberg
The prime minister who promised she would be strong and stable is instead at the top of a party that looks weak and chaotic at a vital time.
She’s in this position, her critics say, because of the choices and missteps she has made.
But her supporters would say it was because for some people in the Tory party who have had years of fury about Europe, nothing would ever have been good enough.
One of her cabinet colleagues is predicting a “long day” but a “solid win”. Let’s see.
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.”
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.