Conservative MPs will vote between 6pm and 8pm to decide whether they still have confidence in Theresa May as their leader.
The result will be announced at 9pm.
Mrs May needs to secure the votes of 158 MPs - half the parliamentary party plus one - to remain as Conservative leader.
If she doesn't then the party will hold an election to choose a new leader.
Theresa May would stay on as Prime Minister for a while even if she loses
But losing this vote wouldn't mean she has to resign as Prime Minister immediately.
In the UK system of Government, the country always has a Prime Minister.
And that means Mrs May will carry on until somebody else is ready to replace her.
In practice, that means that she stays in Number 10 until the Conservatives have chosen a new leader.
If she insisted in resigning early then nobody can stop her. The Queen would probably need to appoint her deputy, the Cabinet Office Minister David Liddington, as a temporary Prime Minister while the leadership election is underway.
But there's no reason to think Theresa May would do this.
How long would a Conservative leadership contest take?
It's thought that the contest could be held over the Christmas break.
However, Theresa May said a new leader would not be in place by the January 21, the deadline for a Commons vote on the Brexit deal.
There are two stages to a Conservative leadership election.
First, MPs choose their two top candidates.
Then, Conservative Party members hold a vote to decide who out of those two contenders becomes their leader.
The new leader will then become Prime Minister
Once a new Conservative leader is in place, they become Prime Minister.
That's because they would be able to command the support of a majority of MPs in the House of Commons.
The Conservatives don't have a majority on their own. They depend on the support of 10 MPs from the DUP, a Northern Ireland party.
But the DUP have already said they will continue to support the Tories regardless of who the party leader is.
The one thing that would change their minds is if a Conservative government succeeded in getting the House of Commons to approve plans for the "backstop" - part of the proposed Brexit withdrawal agreement, which the DUP firmly oppose.
However, it looks very unlikely that the Commons will ever support the backstop as it currently stands.
Theresa May could resign even if she wins the confidence vote
If Theresa May wins the confidence vote than she can continue as party leader. Under Conservative party rules, a new vote can't be held for 12 months.
But it's possible she might feel obliged to resign anyway.
For example, if 100 Tory MPs vote against her then, according to the rules, she can carry on - but she might feel that so many of her own MPs oppose her that she can't really get anything done.
Labour could also call a House of Commons confidence vote
The Conservative confidence vote is a separate process to a confidence vote in the Commons as a whole.
A Commons vote would happen if Labour put down a motion saying they have no confidence in the Government.
And if the Government loses this vote then there will probably be a general election. As far as this vote is concerned, it makes no difference whether Theresa May is Tory leader or someone else.
