Boris Johnson took office as British prime minister today after an audience with Queen Elizabeth, a moment that put an avowed Brexiteer in control of Britain's exit from the European Union for the first time since the shock 2016 referendum.

-Johnson is Britain's new PM, set to appoint ministers

- Brexit architect Cummings to be senior adviser

-New leader vows to quit EU by end of October, come what may

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Johnson enters Downing Street at one of the most perilous junctures in post-World War Two British history - the United Kingdom is divided over its divorce from the EU and weakened by the three-year political crisis that has gripped it since that referendum.

One of Britain's most prominent Brexit campaigners, Johnson has repeatedly pledged to leave the European Union by October 31 - "do or die" - and to inject a new optimism and energy into the divorce, which he argues will bring a host of opportunities.

But his Brexit strategy sets the United Kingdom up for a showdown with the European Union and thrusts it towards a potential constitutional crisis, or an election, at home.

"We are going to get Brexit done on October 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 yesterday after he was elected by Conservative Party members.

To implement Brexit, Johnson will appoint Dominic Cummings, the campaign director of the official Brexit Vote Leave campaign, as a senior adviser in Downing Street.

Today's events combined arcane British political choreography with the realpolitik of appointing a new government - likely to be heavy on Brexit supporters.

Prime Minister Theresa May left Downing Street after a three-year premiership that was marred by crises over Brexit. She formally tendered her resignation to Queen Elizabeth.

May appeared to be fighting back tears as she was applauded out of the House of Commons chamber. Her finance minister, Philip Hammond, resigned.

Johnson had a possible foretaste of turmoil ahead when, as he drove to his audience with the queen, Greenpeace protesters tried - but failed - to block the path of his car as his chauffeur drove around them.

The queen requested he form an administration. His formal title is now "Prime Minister and First Lord of the Treasury".

He is due to enter Downing Street shortly and is expected to give a speech before appointing key members of the government - names that will give a hint of how he will handle Brexit, Britain's most significant decision in decades.

"Boris will build a cabinet showcasing all the talents within the party that truly reflect modern Britain," a source close to Johnson said.

BREXIT GOVERNMENT?

But 'Prime Minister Johnson' - a man known for his ambition, mop of blonde hair, flowery oratory and a cursory command of detail - must solve a series of riddles if he is to succeed where May failed.

The 2016 Brexit referendum showed a United Kingdom divided about much more than the European Union, and has fuelled soul-searching about everything from secession and immigration to capitalism, empire and modern Britishness.

The pound is weak, the economy at risk of recession, allies are in despair at the Brexit crisis and foes are testing Britain's vulnerability.

His party has no majority in parliament so the Conservatives only govern with the support of 10 lawmakers from the Brexit-backing Democratic Unionist Party in Northern Ireland.

While Johnson said he does not want an early election, some lawmakers have vowed to thwart any attempt to leave the EU without a divorce deal. Brexit Party leader Nigel Farage said he was open to an electoral pact with Johnson.

Investors are braced to see who will be handed the top jobs such as finance minister, foreign secretary and Brexit minister.

The appointment of Cummings, known for his campaign skills but also for a combative style that challenges the consensus, indicates Johnson is serious about going in hard on Brexit and wants a first-class political campaigner close.

Cummings, who graduated with a top degree from Oxford, says Britain's political system is broken and Brexit negotiations have been botched by officials and politicians who have thwarted the will of the people.

Interior minister Sajid Javid is widely tipped to stay in a top job - possibly as finance minister - and was spotted flanking Johnson as he arrived before lawmakers.

There is talk that Johnson will appoint career diplomat David Frost as European Union sherpa and adviser on Europe.

A record number of ethnic minority politicians are expected to serve as ministers including Priti Patel, the former aid minister who resigned in 2017 over undisclosed meetings with Israeli officials, and employment minister Alok Sharma.

Foreign Secretary Jeremy Hunt, Johnson's rival for the leadership, was offered the job of defence minister, but turned it down, Sky reported.

NO DEAL?

Johnson has pledged to negotiate a new Brexit deal with the EU before Oct. 31 but if the bloc refuses, he has promised to leave without a deal on Halloween.

That, many investors warn, would send shock waves through the world economy and tip the world's fifth largest economy into recession or even chaos.

Brexit without a divorce deal would roil financial markets and, some bankers warn, weaken London’s position as the pre-eminent international financial centre.

Brexit supporters say those fears are overblown and the United Kingdom will thrive if cut loose from the European project which they cast as a German-dominated bloc that is falling far behind its global competitors such as the United States and China.

"If he really wants a no-deal, he will get it. We will never push an EU member out but we can’t stop him. More likely, his own parliament would," one EU diplomat said.