LONDON (Reuters) - Boris Johnson promised in his first speech as prime minister to lead Britain out of the European Union on Oct. 31 with “no ifs or buts” and warned that if the bloc refused to negotiate then there would be a no-deal Brexit.

Johnson, who has been hailed by U.S. President Donald Trump as Britain’s Trump, is sending the strongest message yet to the EU that he will be taking a distinctly tougher approach to negotiating a revision of the Brexit divorce deal.

He took over from Theresa May at one of the most perilous junctures in post-World War Two British history - the United Kingdom is divided over Brexit and weakened by the three-year political crisis that has gripped it since a 2016 referendum vote to leave the bloc.

“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,” Johnson, 55, said after arriving at the premier’s official residence, No.10 Downing Street.

“We can do a deal without checks at the Irish border,” he said. “It is of course vital at the same time that we prepare for the remote possibility that Brussels refuses any further to negotiate and we are forced to come out with no deal.”

Just hours after arriving in Downing Street, the new Conservative Prime Minister began work with one of the biggest culls of senior government jobs in recent British history, changing all of the main ministers. Most of his appointees were Brexit supporters.

Johnson’s bet is that the threat of a no-deal Brexit will persuade the EU’s biggest powers - Germany and France - to agree to revise the divorce deal that May agreed last November but failed to push through the British parliament.

The gambit, an admission that three years of Brexit talks have failed, sets the United Kingdom up for a showdown with the EU and thrusts Johnson towards a potential constitutional crisis, or an election, at home.

While his strategy could get support from Trump, who had advised May to take a much tougher line with Brussels, he has just 99 days to renegotiate and ratify the so called Withdrawal Agreement that the EU has repeatedly refused to amend.

European Council President Donald Tusk wrote to Johnson saying he was looking forward to discussing cooperation “in detail”.

GRAPHIC: Race to the Prime Ministership - tmsnrt.rs/2Insb9A

‘NEVER MIND THE BACKSTOP’

One of Britain’s most prominent Brexit campaigners, Johnson has repeatedly pledged to leave the EU by Oct. 31 - “do or die” - and to inject a new optimism and energy into the divorce, which he argues will bring a host of opportunities.

In his speech on Wednesday, he rebuked “gloomsters” and the political class who he said had forgotten the people they should serve, promising instead to serve the British people who he said were his government’s true bosses.

One of the issues that prevented May getting a divorce deal through parliament was the Irish “backstop” - an insurance policy designed to prevent the return of a hard border between the Irish Republic and the British province of Northern Ireland.

Johnson said that issue could be dealt with.

“Never mind the backstop. The buck stops here,” he said, adding that new deal could be done that allowed for no border checks. He said the backstop was “anti-democratic”.

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He promised to accelerate preparations for a “no-deal” though he said Britain did not want such an exit.

“Listening to what he said today, I got the impression that he wasn’t just talking about deleting the (Northern Ireland) backstop, he was talking about a whole new deal - a better deal for Britain,” Irish Prime Minister Leo Varadkar said.

“That is not going to happen.”

Many investors say a no-deal Brexit would send shock waves through the world economy, tip Britain’s economy into a recession, roil financial markets and weaken London’s position as the pre-eminent international financial centre.

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“To all those who continue to prophesy disaster, I say yes - there will be difficulties though I believe that with energy and application they will be far less serious than some have claimed,” Johnson said in his speech, watched by his girlfriend, Carrie Symonds, and his staff.

PM JOHNSON

Declared new leader of the Conservative Party on Tuesday after a vote by party members, Johnson began forming his government from Brexit supporters.

Sajid Javid, the 49-year-old son of Pakistani Muslim immigrant parents, was named as his finance minister. Priti Patel was appointed interior minister. Dominic Raab was appointed foreign minister. Stephen Barclay remained as Brexit minister. Ben Wallace was made defence minister.

“He’s severing all connections with the Theresa May regime,” said Jonathan Tonge, professor of politics at the University of Liverpool.

Liz Truss was appointed trade minister, while Gavin Williamson, former defence minister, returned to government as education minister.

Johnson is due to appoint Dominic Cummings, the campaign director of the official Brexit Vote Leave campaign, as a senior adviser in Downing Street.

There was a moment of drama in Johnson’s passage to power as he drove to Buckingham Palace for his audience with Queen Elizabeth and his formal appointment. Greenpeace protesters tried - but failed - to block the path of his car as his chauffeur drove around them.

Earlier May appeared to be fighting back tears as she was applauded out of the House of Commons chamber.

DISUNITED KINGDOM

Johnson, man known for his ambition, messy blond hair, flowery oratory and 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 EU, and has fuelled soul-searching about everything from immigration to capitalism, the legacy of 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.

Johnson’s Conservatives have no majority in parliament and so can govern only with the support of 10 lawmakers from the Brexit-backing Democratic Unionist Party in Northern Ireland.

While Johnson has said he does not want an early election, some lawmakers have promised to block any attempt to leave the EU without a divorce deal.