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 fairly quickly after the voting finishes.
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”.