UK Prime Minister Theresa May will face a vote of no confidence in her leadership later on Wednesday (12).
Conservative MPs will vote between 18:00 GMT and 20:00 GMT.
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 vote 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 the leader of the party would be expected to be prime minister.