Johnson, who is the 14th British prime minister to serve under Queen Elizabeth II, was briefly delayed after climate protesters formed a human chain in front of his motorcade.

After being formally appointed, Johnson promised to deliver Brexit on October 31 “no ifs, no buts” and prove wrong “the doubters, the doomsters, the gloomsters”.

Speaking outside his Downing Street office, Johnson – one of the key figures behind the 2016 Brexit referendum – promised to do a “new deal” with Brussels.

Britain’s polarising new PM

Known for his jokes and bluster, Johnson has sold himself as the optimistic leader to end the Brexit crisis, but his tendency to play to the crowd has drawn accusations of divisive, Trump-style populism.

From his early years in journalism writing what critics called “Euro-myths”, to editorials comparing burqa-wearing Muslim women to “letterboxes”, Johnson has been nothing if not controversial.

The former London mayor has in the past diverted criticism with a witty aside or an absurd anecdote. But the 55-year-old is now taking over as prime minister at one of the most difficult times in Britain’s history.

With his easygoing manner, unpredictability and willingness to laugh at himself, Johnson cuts a vastly different figure from his determined but colourless predecessor, Theresa May.

For supporters, his star power is a political asset.

They hope his maverick approach will help him break the political deadlock over Britain’s exit from the European Union that has left the country in limbo.

Having run cosmopolitan London for eight years, Johnson also says he can unite his Conservative party and the country, both of them still deeply divided over the 2016 referendum vote for Brexit.

But he is a polarising figure, still hated by many voters as the figurehead of a referendum campaign marked by exaggerated claims about the EU and anti-immigrant rhetoric.

His threat to leave the EU without a deal has prompted alarm on both sides of the Channel, sparking accusations that he is not being honest with voters about the economic consequences.

Despite his own socially liberal views – he has long backed gay marriage and advocated an amnesty for illegal migrants as mayor – Johnson has also drawn accusations of “dog-whistle” politics.

Over the years as a journalist he has written articles referring to gay “bumboys”, the Commonwealth’s “flag-waving piccaninnies” and as recently as last summer, derogatory remarks about veiled Muslim women.

His biographer Andrew Gimson said Johnson was not instinctively divisive but delighted in shocking the political establishment – not unlike US President Donald Trump, who is a fan of Britain’s new leader.

Like Trump, Johnson is also accused of being uninterested in the fine detail of policy – “winging it”, as one former colleague described it – leading to some serious mistakes

‘Just Boris’

Born in New York in 1964, into a close and high-achieving family, Alexander Boris de Pfeffel Johnson has always been ambitious.

His first tilt at the Tory leadership in 2016 failed after his closest ally betrayed him, but he laid the groundwork for this contest early.

Johnson resigned as May’s foreign minister a year ago in protest at her EU strategy, cementing his position as the champion of Brexit.

He ran a disciplined campaign, only briefly derailed by headlines about a police visit to his home following a noisy row with his girlfriend.

It reignited interest in the love life of a serial philanderer who has married twice and has an unknown number of children.

But Johnson never comments on his private life and for many supporters, as with other controversies, the row made little difference – it was just “Boris being Boris”.

Educated at the elite Eton school and Oxford University, Johnson started his professional life as a journalist for The Times but was sacked for fabricating quotes.

He later made a name for himself as Brussels correspondent for The Daily Telegraph, peddling factually-debatable stories about EU regulations.

Johnson won nationwide fame as a guest on the satirical TV talk show “Have I Got News For You”, where his bumbling, aristocratic persona made him a hit.

He became an MP in 2001, but was sacked as conservative party arts spokesman in 2004 for allegedly lying about an extra-marital affair.

Foreign ministry slip

In 2008, harnessing his celebrity status to reach across the political divide, he was elected as mayor of multi-ethnic, Labour-voting London.

He was re-elected in 2012 and oversaw the London Olympic Games that year, memorably getting stuck on a zip-wire while celebrating Britain’s first gold medal.

With his eye on Downing Street, Johnson returned to parliament in 2015 as MP for a London suburb.

He promised to oppose the expansion of Heathrow Airport, but later as foreign minister, he missed a key parliamentary vote on the project after hastily arranging a trip to Afghanistan.

While he often highlights how he cut crime, invested in transport and housing as mayor, Johnson is less effusive about his time at the Foreign Office.

He was accused of jeopardising the case of Nazanin Zaghari-Ratcliffe, a British-Iranian woman held in Tehran for sedition, by mischaracterising her job.

The Chatham House think-tank was scathing about his record, saying: “Where gravitas and grasp of detail were needed, Johnson supplied bon mots.”