The 55-year-old will assume the role later on Wednesday, just over three months before he has to make good on his promise to lead the UK out of the European Union – a process known as Brexit – by October 31 with or without a withdrawal agreement.
He also must win over the many Britons who oppose Brexit. In his victory speech on Tuesday, Johnson vowed to deliver Brexit and unite the country.
The former foreign secretary will replace Theresa May, who announced her resignation last month after parliament repeatedly rejected the withdrawal agreement she struck with the 28-nation bloc.
The departing leader will attend the weekly prime minister’s questions period in the House of Commons for the last time on Wednesday. May will then head to Buckingham Palace to resign and ask Queen Elizabeth II to invite Johnson to form a new government.
The 93-year-old monarch will then meet Johnson – the 14th prime minister of her 67-year reign – before the new leader stands outside 10 Downing Street to address the nation before announcing the names of his team either late on Wednesday or early on Thursday.
Johnson’s office said his government would be a “cabinet for modern Britain” with more women and a record number of ministers from ethnic minorities.
Many members of his administration will be strong Brexit supporters such as Johnson, but he is also set to include some pro-EU politicians.
Johnson, who is also a former mayor of London, won the contest to lead the Conservatives – or Tories – by securing the votes of 92,153 members of the ruling party, while his rival, Foreign Secretary Jeremy Hunt, won 46,656.
Iain Dale, a political commentator, told Al Jazeera that Johnson has to deliver Brexit on October 31 “do or die”, prompting him to demand a pledge of embracing the possibility of “no-deal Brexit” from his cabinet members.
“If he doesn’t achieve that, he will be one of the shortest-serving Tory leaders, because the Conservative Party is very vicious. If it sees that someone’s failed, it will get rid of him,” he said.
A number of Conservative Party ministers have already announced they will resign to fight against a no-deal Brexit, which experts warn would push the UK’s economy into a severe downturn.
Business and political observers are bracing to see who will be handed the top jobs such as finance minister, foreign secretary and Brexit minister.
Interior Minister Sajid Javid is widely tipped to stay in a top job, possibly as finance minister.
There is talk that Johnson will appoint career diplomat David Frost as EU sherpa and adviser on Europe.
Dominic Cummings, the Brexit-supporting campaign director of Vote Leave, will be a senior adviser, a source for the new leader’s team said.
The cabinet is also expected to include Priti Patel, the former aid minister who resigned in 2017 over undisclosed meetings with Israeli officials, and Employment Minister Alok Sharma.
Hunt, Johnson’s rival for the leadership, was offered the job of defence minister, but he turned it down, Sky broadcaster reported.