Theresa May's statement before the vote - in full
The British prime minister says she will contest a vote of no confidence to "deliver the Brexit people voted for".
Theresa May 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 between 18:00 GMT and 20:00 GMT.
Mrs May said changing Conservative leader would "put our country's future at risk and create uncertainty when we can least afford it".