Only one person, the joke doing the rounds in Parliament goes, can stop the dishevelled, blond-haired, crowd-pleasing former foreign secretary, Boris Johnson, from becoming the country’s next prime minister.

That is Mr. Johnson himself.

One of Britain’s most recognisable, and now most divisive politicians, Mr. Johnson has a history of verbal gaffes, a poor record as a minister and many enemies in Parliament, not to mention among the voters who reject Brexit, which he helped persuade Britons to embrace in a 2016 referendum.

But his charisma, flair for publicity and record of winning two elections as mayor of London make him the runaway favorite in a crowded field to succeed Prime Minister Theresa May, according to bookmakers.

A victory for Mr. Johnson would have significant repercussions and could increase the prospects of Britain hurtling out of the European Union without an agreement at the end of October, despite the potentially dire economic consequences.

It also sets up a possible constitutional showdown with Parliament, which has shown that the one thing it can agree on is that there should never be a no-deal Brexit. If Britain’s new prime minister — whoever it is — was willing to exit the bloc without a deal, it is unclear if Parliament would have the power to stop the move, according to constitutional scholars.

“A new leader will have the opportunity to do things differently and have the momentum of a new administration,” Mr. Johnson said on Friday, according to Reuters, illustrating why Britain could be headed into turbulent waters. “We will leave the E.U. on October 31, deal or no deal. The way to get a good deal is to prepare for a no deal.”

Not only is Mr. Johnson a firm proponent of Brexit — albeit one who is instinctively more flexible than Mrs. May — but several European Union leaders have hinted they regard the idea of dealing with him as a nightmare come true. They likely will be loath to make him any concessions.

As a child Mr. Johnson famously announced his ambition to become “world king,” and even if the job he now seeks is not quite that, the stars could be aligning for him.

The results of the European Parliament elections are to announced on Sunday, and his Conservative Party, according to opinion polls, appears headed to a humiliating defeat at the hands of the populist Euroskeptic, Nigel Farage, and his new Brexit Party, which has been fired up by Mrs. May’s failure to take Britain out of the European Union on March 29 as planned.

If the takeaway for the Tories from the expected election drubbing is that they need to fight fire with fire, that would point overwhelmingly to Mr. Johnson.

“He is the ‘In case of emergency, break the glass’ candidate,” said Tim Bale, a professor of politics at Queen Mary University of London. “The fire has begun and the Conservative Party is genuinely worried. That makes them more inclined to pick someone with the X factor.”

“It’s fair to say that it’s his to lose,” Mr. Bale said of the leadership contest, before noting: “But, of course, he is perfectly capable of losing it.”

There is no shortage of other contenders should Mr. Johnson falter.

The field is crowded because Conservative lawmakers use leadership contests to raise their profiles and put down markers for the future, or to amass a bloc of votes that they can use in bargaining with front-runners for future jobs.

Under its leadership rules, Conservative Party lawmakers will whittle down probably around a dozen candidates to a shortlist of two. Party members, who are thought to number around 120,000, mainly aging, activists, will then choose the winner.