Here are some of the initial reactions from home and abroad to Johnson's victory:
- 'He will be great' -
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"Congratulations to Boris Johnson on becoming the new Prime Minister of the United Kingdom. He will be great!" US President Donald Trump tweeted.
Trump has declared himself a big fan of Johnson.
Last week he predicted Johnson would fix what Trump called the "disaster" that outgoing Conservative Prime Minister Theresa May had triggered in trying to lead Britain out of the European Union.
"He's a different kind of a guy, but they say I'm a different kind of a guy too. We get along well. I think we'll have a very good relationship," Trump told reporters.
- 'Challenging times ahead' -
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"Congratulations to Boris Johnson for being nominated as prime minister," the incoming head of the European Commission, Ursula von der Leyen said in Paris. "I am looking forward to having a good working relationship with him.
"There are many different and difficult issues to tackle together. We have challenging times ahead of us."
"We look forward to working constructively with PM Boris Johnson when he takes office, to facilitate the ratification of the Withdrawal Agreement and achieve an orderly Brexit," EU negotiator Michel Barnier tweeted.
"We are ready also to rework the agreed declaration on a new partnership in line with EUCO guidelines," Barnier said, referring to the non-binding political plan for future EU-UK ties that May signed in November.
Outgoing European Commission president Jean-Claude Juncker also sent his congratulations to Johnson, according to a spokeswoman.
"The president wants to work with the prime minister in the best way possible and beyond this, I'm going to reserve my comments as the news has just freshly broken."
- Iran 'not seeking confrontation' -
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"I congratulate my former counterpart, @BorisJohnson on becoming UK PM," Iranian Foreign Minister Mohammad Javad Zarif tweeted amid a standoff between the two countries over the seizure of oil tankers.
"Iran does not seek confrontation. But we have 1500 miles of Persian Gulf coastline. These are our waters & we will protect them," he said.
- 'Coordinate closely ' -
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"I congratulate Boris Johnson and I will call him when he is officially prime minister," French President Macron said.
"I want very much to work with him as quickly as possible and not just on European subjects and the continuation of negotiations linked to Brexit, but also on international issues on which we coordinate closely with Britain and Germany... like the situation in Iran."
- 'The bankers' friend' -
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Boris Johnson has won the support of fewer than 100,000 unrepresentative Conservative Party members by promising tax cuts for the richest, presenting himself as the bankers' friend, and pushing for a damaging No Deal Brexit," said Jeremy Corbyn, leader of the main opposition Labour Party.
"But he hasn't won the support of our country," he said on Twitter.
"Johnson's No Deal Brexit would mean job cuts, higher prices in the shops, and risk our NHS being sold off to US corporations in a sweetheart deal with Donald Trump.
"The people of our country should decide who becomes the Prime Minister in a General Election."