* Results due at 1200 GMT
* Candidates need 17 votes to go through
* Johnson way ahead of rivals - betting odds
LONDON, June 13 (Reuters) - The contest to replace British Prime Minister Theresa May is due to narrow on Thursday when Conservative Party lawmakers vote to knock out at least one of 10 candidates in the first ballot, though Boris Johnson is far ahead of rivals.
The United Kingdom could be heading towards a constitutional crisis over Brexit as many of the candidates vying to succeed May are prepared to leave the European Union on Oct. 31 without a deal but parliament has indicated it will try to thwart such a scenario.
Nearly three years since the United Kingdom voted 52%-48% to leave the EU, the ruling Conservative Party had its worst result in centuries in a European election last month, and opinion polls indicate a snap election would produce a hung parliament.
Conservative lawmakers cast their votes in booths in a room in the British parliament on Thursday between 0900 GMT and 1100 GMT. Any candidate with 16 votes or fewer is eliminated. If all candidates have more than 16 votes, the one with the fewest votes is eliminated.
The second round is due on June 18 with further ballots planned for June 19 and June 20 until there are just two candidates. A postal ballot of the wider Conservative Party membership will then be held to pick a leader. A new prime minister should be chosen by the end of July.
Johnson, who led the official campaign to leave the EU in the 2016 referendum, is the frontrunner to replace May despite a long record of scandals and gaffes. Betting markets give him a 67% probability of winning the top job.
His closest rivals are Jeremy Hunt, currently foreign minister, and Andrea Leadsom, former leader of the House of Commons, which the betting markets give around a 10% probability of winning.
Of 313 Conservative Party lawmakers, 81 have publicly backed Johnson, 32 have publicly backed Michael Gove, the environment minister, and 30 have publicly backed Hunt, The Daily Telegraph said.
Johnson kicked off his official campaign on Wednesday with a pledge to lead Britain out of the European Union on Oct. 31 and a warning to his divided Conservative Party that “delay means defeat”.
“After three years and two missed deadlines, we must leave the EU on October 31,” Johnson, a 54-year-old former foreign minister and London mayor, said. “I am not aiming for a no-deal outcome.”
Johnson, whose unconventional style has helped him shrug off a series of scandals in the past, has won over much of his party by arguing that only he can rescue the Conservatives by delivering Brexit.
The EU has refused to renegotiate the Withdrawal Agreement reached with May last November, and Ireland has indicated it is not willing to change the Irish border “backstop” that upset the Northern Irish party which props up May’s minority government.
Johnson made his name as an EU-bashing journalist in Brussels before entering politics - and upset some Europeans before Britain’s Brexit referendum by comparing the EU’s goals with those of Hitler and Napoleon.
Michael Spencer, a financier and Conservative Party donor who is supporting six of the candidate, told the BBC that Johnson could make a “very decent prime minister.”
“He is certainly a person who has passion, energy, and has a narrative and has popular support behind him, which is kind of essential for the Conservative Party and the country as a whole,” Spencer said. (Writing by Guy Faulconbridge; Editing by Janet Lawrence)