British Conservative MPs are voting for the first time (from 1000GMT) for a new leader who will also become Prime Minister.
Home Secretary Theresa May is favourite to win the process which the party says will conclude by September 9th.
The candidate who comes last in Tuesday's vote by MPs will be eliminated, with further rounds due later this week and next.
It comes after Prime Minister David Cameron decided to resign after Britons chose to leave the European Union in a referendum last month.
Political reporter Theo Usherwood says the process could wrap up before September.
IN: "It's an…
OUT: …stand aside."
DUR: 45 seconds