A former candidate for Britain's Tory leadership says there's no guarantee Boris Johnson will become Britain's next prime minister.

Rory Stewart came from the back of the pack to take fourth place in the Tory leadership race.

Either Boris Johnson or Jeremy Hunt will be named the winner on July 23rd.

Stewart told Mike Hosking the eccentric behaviour that made Johnson famous, could also prevent him from taking the top job.

"Boris is the clear leader and it's really his to lose, but he has had some pretty eccentric performances lately in the media."

He says that Hunt is a very steady and experienced Minister. Stewart says that voters will have to pick between the more colourful figure or the quieter, more dignified candidate.

Stewart says he would be honoured to serve under Hunt as a Minister, but he would decline an such offer to join Johnson's cabinet.

However, regardless of who wins, Stewart says that the Conservative Party will be under intense strain.

"Boris Johnson has promised completely incompatible. He told the more pro-European centrist group he is going to go for a soft Brexit, and he told the hard Brexiteers that he's going to go for a hard Brexit." 

Stewart says that disappointing both sides would lead to a general election, that the Conservatives would not win without having delivered Brexit.