Boris Johnson has become Britain's Prime Minister, installing allies and Brexiteers in his new Cabinet in a clean-out dubbed a "summer's day massacre" by one MP.

Key points: The Queen formally invited Mr Johnson to become Britain's latest PM

The Queen formally invited Mr Johnson to become Britain's latest PM He spoke of an ambitious agenda outside 10 Downing Street

He spoke of an ambitious agenda outside 10 Downing Street He began with a cull of ministers described as a "massacre"

The new British Prime Minister visited the Queen on Wednesday afternoon (local time) to receive her formal blessing for his ascension to the position.

He returned to Number 10 Downing Street to deliver his first address as PM, just a few hours after his predecessor Theresa May issued her final remarks, in which she warned of the "heavy responsibilities" of the job.

"Do not underestimate this country," he said, adding that Britons needed to cast off their self-doubt.

Each British Prime Minister must seek the approval of their sovereign in order to form Government. ( AP: Victoria Jones, pool )

"The people who bet against Britain are going to lose their shirts because we are going to restore trust in our democracy, and we are going to fulfil the repeated promises of parliament to the people and come out of the EU on October 31, no ifs or buts."

He derided "the doubters, the doomsters, the gloomsters" who believe he will fail to break the Brexit impasse that defeated Ms May.

Mr Johnson easily defeated Conservative rival Jeremy Hunt in the vote to become the new leader of the Conservative Party — and, by default, Britain's PM — winning two-thirds of the votes of about 160,000 party members across the UK.

His motorcade was interrupted on its way to Buckingham Palace by Greenpeace protesters, who unfurled a banner warning of the climate emergency facing the planet.

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A handful of protesters formed a human chain across The Mall, forcing the motorcade to come to an abrupt halt and police motorcycle outriders to force them out of the way.

On hand to welcome him to Number 10 were various staff members and his partner Carrie Symonds.

'The buck stops here'

Mr Johnson's girlfriend waited with a welcome party outside 10 Downing Street. ( Reuters: Hannah McKay )

Mr Johnson enters Downing Street at one of the most perilous junctures in post-World War II British history.

The UK is divided over its divorce from the EU and weakened by the three-year political crisis that has gripped it since that referendum.

But his strategy sets the UK up for a showdown with the EU and thrusts it towards a potential constitutional crisis, or an election, at home.

One of the issues that prevented Ms May from getting a divorce deal through Parliament was the Irish "backstop" — a provision that would maintain a customs union with the EU and Northern Ireland if no better solution was found.

"Never mind the backstop. The buck stops here," he said.

Chief EU Brexit negotiator Michel Barnier said "we are ready to listen and to work with" the new PM, but did not budge on the bloc's refusal to alter the deal.

"A no-deal Brexit will never be, never, the choice of the EU. But we are prepared," he said in Brussels.

Mr Johnson promised to deliver Brexit "in a new spirit of can do".

"I say to all the doubters: 'Dude, we are going to energise the country, we are going to get Brexit done,'" he said.

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To succeed, the new Prime Minister must win over the many Britons opposed to Brexit.

In a sign he hopes to move beyond the largely white, male and affluent Conservative members who chose him as their leader, Mr 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.

British Prime Minister Boris Johnson is due to design his maiden cabinet later on Wednesday (local time). ( Reuters: Hannah McKay )

Cull was swift and brutal

Mr Johnson wasted no time in clearing space in his Cabinet for the true believers.

One Tory, Nigel Evans, described the speed of the cull as a "summer's day massacre".

Trade Secretary Liam Fox was sacked, and so too was Defence Secretary Penny Mordaunt — lasting just 85 days as the first female in the role.

Jeremy Hunt, Mr Johnson's rival for the prime ministership, declined to serve in his administration as Defence Secretary, but offered the victor his "full support".

They followed the resignation of the Chancellor of the Exchequer Philip Hammond, whose resignation letter warned major spending initiatives were contingent on a deal with Brussels.

Others resigned on principle too, including Justice Secretary David Gauke, International Development Secretary — and leadership hopeful — Rory Stewart, and the Minister for Cabinet David Lidington.

Also demoted to the backbench was Northern Ireland Secretary Karen Bradley, Secretary of State for Housing, Communities and Local Government James Brokenshire, Transport Secretary Chris Grayling, Education Secretary Damian Hinds, Business Secretary Greg Clark and Immigration Secretary Caroline Nokes.

Out too was David Mundell, whose portfolio covered Scotland.

Among those appointed to major portfolio positions include prominent Brexiteers Dominic Raab as Foreign Secretary and First Secretary of State, and Priti Patel — who was sacked as International Development Secretary in 2017 for secret meetings with the Israel government — as Home Secretary.

Former Home Secretary Sajid Javid, a one-time leadership rival of Mr Johnson who ended up supporting him, is the new Chancellor of the Exchequer.

Britain's former home secretary Sajid Javid is the new Chancellor of the Exchequer. ( AP: Frank Augstein, File )

One of his senior advisers is set to be Dominic Cummings, lead strategist for the "Vote Leave" campaign in the 2016 EU membership referendum.

A contentious figure, Mr Cummings was found to be in contempt of Parliament earlier this year for refusing to give evidence to a committee of MPs investigating "fake news."



ABC/wires