Boris Johnson, the Brexiteer who has promised to lead Britain out of the European Union with or without a deal by the end of October, will replace Theresa May as prime minister after winning the leadership of the Conservative Party on Tuesday.
His convincing victory catapults the United Kingdom toward a showdown with the EU and toward a constitutional crisis at home, as British lawmakers have vowed to bring down any government that tries to leave the bloc without a divorce deal.
Johnson, the face of the 2016 Brexit referendum, won the votes of 92,000 members of the Conservative Party, almost twice as many as his rival, Foreign Secretary Jeremy Hunt.
May will leave office on Wednesday after going to Buckingham Palace to see Queen Elizabeth, who will formally appoint Johnson.
"We are going to get Brexit done on Oct. 31, and we are going to take advantage of all the opportunities it will bring in a new spirit of 'can do'," Johnson, 55, said after the result was announced.
"Like some slumbering giant, we are going to rise and ping off the guy-ropes of self-doubt and negativity."
Johnson said the mantra of his leadership campaign had been to "deliver Brexit, unite the country and defeat (opposition Labour leader) Jeremy Corbyn - and that is what we are going to do".
US President Donald Trump quickly tweeted his congratulations, saying: "He will be great!".
The two have recently been complimentary about each other - yet Trump is one of the many leaders, including Russian President Vladimir Putin and Turkish President Tayyip Erdogan, who have been the subject of derogatory remarks by Johnson.
In 2015, he accused Trump, then a candidate for office, of "stupefying ignorance" that made him unfit to be president.
An avowed Brexit supporter will now lead the government for the first time since the United Kingdom voted to leave the EU.
The pound has fallen sharply in recent weeks on concerns about a "no-deal" Brexit. The investment bank Goldman Sachs raised its probability of a no-deal Brexit to 20% from 15%, but kept the odds of a negotiated Brexit deal unchanged at 45%.