Brexit hardliner Boris Johnson has won the race to lead Britain's governing Conservative Party, and will become the country's next prime minister.
He defeated his rival Jeremy Hunt overwhelmingly in a vote of Conservative Party members.
His victory catapults the United Kingdom towards a Brexit showdown with the EU and towards 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.
Theresa May will leave office on Wednesday after going to Buckingham Palace to see Queen Elizabeth, who will formally appoint Johnson before he enters Downing Street.
The victory is a triumph for the 55-year-old Johnson, an ambitious but erratic politician whose political career has veered between periods in high office and spells on the sidelines.
Johnson has vowed that Britain will quit the European Union, "come what may," on the scheduled Brexit departure date of October 31 even if it means leaving without a divorce deal.
But he faces a rocky ride from a Parliament determined to prevent him from taking the UK out of the bloc without a withdrawal agreement.
Johnson said as prime minister he will "deliver Brexit, unite the country" and defeat the Labor opposition.
In a brief speech Tuesday meant to rally the party faithful, Johnson sought to radiate optimism following his victory and promised to deliver Brexit.
Within half an hour of his victory, US President Donald Trump had tweeted his congratulations, adding: "He will be great!"
Michel Barnier, the European Union's chief Brexit negotiator, said he looks forward "to working constructively" with Boris Johnson.
He added he is looking to push through the ratification of the withdrawal agreement negotiated by May, but said nothing about Johnson's claim that the deal must be renegotiated.
Barnier said the EU would be willing to adapt a political text which goes alongside the withdrawal agreement.
EU leaders have long said that they will not reopen the 585-page legal text.
"We are ready also to rework the agreed Declaration on a new partnership," said Barnier.
The rise of Alexander Boris de Pfeffel Johnson, often referred to as simply "Boris", to the top of British politics is the grandest twist so far in a career that has taken him from journalism via TV-show fame, comedy and scandal into the political brinkmanship of Britain’s Brexit crisis.
Born in New York, Johnson was educated at Eton, Britain’s most exclusive school, and at Balliol College, Oxford. He began his career at a management consultancy in the City of London but dropped out after a week.
He then turned to journalism, but was sacked from the Times newspaper for making up quotes.
Hired by the Daily Telegraph, Johnson infuriated European officials and delighted then-prime minister Margaret Thatcher by lampooning the European Economic Community with a host of sometimes misleading reports from Brussels.
After entering politics, he was sacked from the Conservative Party’s policy team while in opposition for lying about an extramarital affair.
But his sometimes shambolic personal appearance and disarmingly self-deprecating confidence have allowed him to survive both gaffes and scandal. He served two terms as London mayor, from 2008 to 2016.
In 2016, he became one of the most recognizable faces of the Brexit campaign, which won the referendum by 52% to 48%.
"I know that there will be people around the place who will question the wisdom of your decision, and there may be some people here who still wonder what they have done," Johnson told party members at the Queen Elizabeth conference center opposite parliament. "I will just point out to you that no one party, no party has a monopoly of wisdom." Johnson said.
"Do you look daunted? Do you feel daunted? I don't think you look remotely daunted to me."
