Former London Mayor Boris Johnson and Home Secretary Theresa May are entering the race to become Conservative Party leader and therefore UK Prime Minister.
They're considered the clear front-runners in the process which will choose a successor to David Cameron.
He's stepping aside after losing a campaign to keep Britain in the European Union.
Whoever becomes Conservative leader will also be Prime Minister and be tasked with negotiation a so-called Brexit.
Political reporter Theo Usherwood says Theresa May has been setting out her stall.
IN: "She's been…
OUT: …Brexit."
DUR: 34 sec
ALT:
A challenge is also expected for Labour leader Jeremy Corbyn.
MP Angela Eagle is thought to be about to announce a challenge with many MPs angry at Mr Corbyn's leadership of the opposition.
Political reporter Theo Usherwood says she may not be the only one.
IN: "Watch out…
OUT: …losing."
DUR: 34 sec