His convincing victory catapults the United Kingdom towards a 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.

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 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".

U.S. President Donald Trump congratulated Johnson, saying he would be great.