UK Prime Minister Theresa May faces a vote of confidence in her leadership later after 48 of her Conservative MPs called for one to be held.
But a defiant Mrs May vowed to contest the vote “with everything I have got”.
She warned that a new prime minister would be faced with the choice of “delaying or even stopping Brexit”.
A majority of Tory MPs have publicly said they will back the PM in the vote, which runs for two hours from 18:00 GMT, but it is a secret ballot.
A result is expected fairly quickly after the voting finishes.
Mrs May is expected to address a meeting of backbench Conservative MPs just before voting begins – and a party spokesman suggested part of her message would be that the vote was not “about who leads the party into the next election”, but about whether now was the right time for a change.
Immediate statements of loyalty for the prime minister were issued by every member of her cabinet, including several who have been touted as possible successors.
Foreign Secretary Jeremy Hunt said Mrs May was “the best person to make sure we actually leave the EU on March 29”, while Home Secretary Sajid Javid said a leadership contest would be seen as “self-indulgent and wrong”.
So far, 174 Tory MPs have publicly said they will vote for her, with 34 publicly against, according to BBC research. She needs to secure the votes of 158 MPs to survive.
If Mrs May wins the confidence vote she cannot be challenged as Conservative leader for at least another year.
If she does not win the vote there would then be a Conservative leadership contest in which she could not stand.
If Mrs May won – but not overwhelmingly – she may decide to stand down as party leader and trigger a leadership contest in which she could not stand.
In her statement delivered early on Wednesday morning, Mrs May said: “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 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”.
“I have devoted myself unsparingly to these tasks ever since I became prime minister and I stand ready to finish the job.”
Not immediately. She would be expected to stay on as a caretaker prime minister until a new Conservative leader is selected by the party, a process that could take up to 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.
As leader of the largest party in the Commons, the new Conservative leader would then be expected to be asked to form a government and become prime minister, without a general election.
Mrs May faced repeated calls from Labour MPs for her to quit – or hold another EU referendum – at a rowdy Prime Minister’s Questions.
Labour leader Jeremy Corbyn said: “The time for dithering and delay is over. The prime minister has negotiated her deal. She has told us it is the best and only deal available.
“There can be no more excuses, no more running away. Put it before Parliament and let’s have the vote.”
Mrs May said the vote will take place, and as Labour MPs shouted “when?” at her she said: “We’ve had a meaningful vote, we had it in in the referendum in 2016.”
The PM added: “And if he wants a meaningful date I’ll give him one; 29 March 2019 when we leave the European Union.”
Veteran pro-European MP Ken Clarke called the prospect of a leadership contest “unhelpful, irrelevant and irresponsible”, prompting loud cheers from the Tory benches.
But the SNP’s leader at Westminster Ian Blackford called on Mrs May to stand down, saying her government was “an embarrassment”.