Maverick candidate Rory Stewart was knocked out of the race to become Britain's next prime minister on Wednesday, as Boris Johnson extended his lead over the three other remaining contenders.

Boris Johnson MP, frontrunner to become Leader of the Conservative Party and the next Prime Minister, leaves home, London, UK, June 19, 2019. [Photo: IC]

International Development Secretary Stewart actually lost votes to come in last in the third ballot of Conservative MPs.

Former foreign minister Johnson, on his 55th birthday, extended his commanding lead with the support of 143 out of 313 members of Britain's governing party.

Foreign Secretary Jeremy Hunt came second with 54, Environment Secretary Michael Gove followed with 51 and Interior Minister Sajid Javid won 38.

The winner will take over from Theresa May as prime minister next month.

His biggest task is to end the political deadlock over Britain's exit from the European Union, currently scheduled for October 31.

Stewart won 27 votes, 10 fewer than he received in Tuesday's second ballot, ending a remarkable run by an unorthodox politician few expected to get this far.

Two further ballots are scheduled for Thursday to choose a final pair that will then be put to 160,000 grassroots party members.

"Thank you once again to friends and colleagues for your support in the third ballot -- especially on my birthday!" tweeted Johnson after Wednesday's result.

"We've come a long way but we have much further to go."

- 'Nobody has plan' -

Initially a rank outsider, Stewart's social media-driven campaign had garnered momentum as the only candidate to rule out leaving the EU without a divorce deal with Brussels.

But following a lacklustre performance in a televised debate on Tuesday night, Stewart shed support.

He said he was bewildered at the loss of votes, but told reporters: "I believe the arguments I made will be vindicated.

"Nobody has a plan for how they are going to get Brexit through parliament. Nobody has a plan on how they're going to renegotiate with the EU. Nobody can define what a no-deal Brexit is.

"These are the fundamental truths that will haunt us for the next six months."

Stewart wanted to try again to get the divorce deal May struck with Brussels through parliament, even though MPs have rejected it three times.

The other candidates say they will threaten to leave with "no deal" in a bid to change the text, which the European Union has so far said will not be reopened.

Johnson and Javid insist Britain must leave on the twice-delayed October 31 exit date, while Hunt and Gove are prepared to delay if a divorce deal with Brussels seems close.

- 'Take on Boris' -

Former Brexit secretary Dominic Raab, who had warned he was willing to suspend parliament if it blocked his "no deal", was eliminated in Tuesday's second ballot and threw his weight behind Johnson.

Hunt has come second in every ballot so far and said he was "best placed to take on Boris".

"If I make it to the final I will put my heart and soul into giving him the contest of his life: in politics today the unexpected often happens," he tweeted.

"The stakes too high to allow anyone to sail through untested."

Home Secretary Javid tweeted: "Delighted to make it through to the final day of MP voting. Grateful for the support of many excellent colleagues. We can do this!"

May officially stepped down as Conservative party leader this month over her failure to deliver Brexit on time, although she remains prime minister until her successor is chosen.

Thursday's fourth ballot takes place between 10:00am and 12:00pm (0900 GMT and 1100 GMT), with the result declared an hour later.

If no candidates have voluntarily withdrawn and three remain, MPs are then scheduled to vote again between 3:30pm and 5:30pm (1430 GMT and 1630 GMT).