British ruling Conservative party leadership and Prime Minister contender Rory Stewart speaks to the media as he leaves television news network studios in London. Picture: Matt Dunham/AP

London - Outspoken former foreign secretary Boris Johnson is expected Wednesday to consolidate his frontrunner status to succeed Theresa May as British prime minister and leader of the ruling Conservatives.

The party's 313 lawmakers will hold a third vote on the five remaining candidates after Brexit hardliner Johnson topped Tuesday's second ballot with 126 votes, far ahead of the 46 for second-placed Foreign Secretary Jeremy Hunt.

In a boost to Johnson, former Brexit secretary and fellow eurosceptic Dominic Raab backed him after his own elimination from the contest in Tuesday's vote.

"I think he's the one that is the most credible to get us out of the EU by the end of October," Raab told broadcaster ITV, referring to the extended deadline for Britain to leave the European Union.

"And above all he's got the optimism, and I think this country needs to feel good about itself again, and I think he's the man to deliver that," said Raab, who came last with 30 votes on Tuesday.

The person with the lowest number of votes on Wednesday will also be eliminated, with two further votes scheduled on Thursday to reduce the field to two candidates for a postal vote by the 160,000 party members.

The final result is expected in the week beginning July 22.

Environment Secretary Michael Gove, who came third on Tuesday with 41 votes, said he was "delighted" to have the support of more lawmakers.

"I have a clear plan to deliver Brexit and take the fight back to [opposition Labour leader Jeremy] Corbyn and those who want to take Britain backwards," Gove tweeted.

"Great to see support moving towards me after last night's BBC debate [between the five remaining candidates]," he wrote.

Hunt told BBC Radio 4 that he is best placed to negotiate Britain's withdrawal from the European Union, saying Britain needs a leader the EU can trust but who is "prepared to walk away" from a deal.

Home Secretary Sajid Javid and International Development Secretary Rory Stewart, a rising centre-ground candidate, also remain in the race.

Stewart tweeted on Wednesday that the contest was developing into "a simple choice" between him and Johnson.

"I am in this to win - the real alternative to Boris," he said, dismissing rumours that he had spoken to Gove about one of them dropping out of the race to support the other.

"I am the candidate with the most momentum, energy and public appeal. Our members deserve a choice," Stewart wrote. "If it's not me, it will be PM Boris."

Johnson had the backing of 39 per cent of Conservative party members questioned by YouGov in a poll conducted last month for The Times newspaper.

May stepped down as Conservative leader on June 7 after failing to win parliamentary approval for the Brexit deal she negotiated with Brussels.