Boris Johnson has been appointed as the new prime minister of the United Kingdom after Theresa May stood down over her failure to lead Britain out of the European Union.

Johnson, 55, was appointed to the premiership by Queen Elizabeth II in a formal meeting at Buckingham Palace on Wednesday.

His elevation to the UK's highest political office was a formality after being announced on Tuesday as the winner of an internal ruling Conservative Party leadership contest involving a ballot of the party's some 160,000 members.

During his campaign, Johnson pledged to renegotiate the withdrawal agreement brokered during months of arduous negotiations between May and EU leaders or leave the bloc on the UK's scheduled departure date of October 31 without a deal.

The withdrawal agreement has already been rejected three times by the UK's parliament, prompting May to announce her resignation in May amid a political impasse.

"We will do a new deal, a better deal, that will maximise the opportunities of Brexit while allowing us to develop a new and exciting partnership with the rest of Europe based on free trade and mutual support," Johnson, a former foreign secretary and mayor of London, said on Wednesday.

"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 added in a speech outside 10 Downing Street, the UK PM's official residence.

Nearly 52 percent of Britons - more than 17 million people - voted to leave the EU during a divisive referendum held in June 2016. Turnout for the poll was more than 72 percent.

'Challenging times ahead'

Despite Johnson's pledges to change the terms of Brexit, EU leaders have repeatedly ruled out renegotiating the withdrawal agreement.

Reiterating the bloc's position on Wednesday, the European Parliament's Brexit Steering Group (BSG) said in a statement that the comments made by Johnson during his campaign to lead the Conservative Party "have greatly increased the risk of a disorderly exit of the UK".

The group added that a no-deal exit would be "economically very damaging, even if such damage would not be inflicted equally on both parties".

The cautionary note came after the EU's pick for the bloc's top job on Tuesday warned Johnson of "challenging times ahead".

Ursula von der Leyen, the German conservative set to take over at the helm of the European Commission from November, also said that all parties had a "duty to deliver something which is good for people in Europe and the United Kingdom".

Johnson wants to scrap the so-called "Irish backstop", an insurance policy written into the Brexit withdrawal deal to maintain an open border for economic and security reasons between the Republic of Ireland and Northern Ireland, a constituent of the UK, until a new British-EU trade deal is reached.

Those in favour of Brexit fear the backstop could trap Britain in EU trading rules indefinitely and prevent the UK from striking trade deals with countries around the world, given that a new trade accord with the bloc could take years to negotiate.

"I am convinced we can do a deal, without checks at the Irish border, because we refuse under any circumstances to have such checks and yet without that anti-democratic backstop," Johnson said on Wednesday, adding that Britain also must be "prepared for the remote possibility that Brussels refuses to negotiate any further".

"I will take personal responsibility for the change I want to see; never mind the backstop, the buck stops here," he said. "The British people have had enough of waiting, the time has come to act, to take decisions … to change this country for the better."

But Irish Prime Minister Leo Varadkar swiftly rebutted Johnson's suggestions, saying his calls for an altered deal were "not in the real world".

"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," Varadkar said. "That is not going to happen," he added.

'No mandate'

Johnson also listed a wide range of domestic policy ambitions, including reforming social care provisions, recruiting 20,000 new police officers, improving the UK's physical and digital infrastructure and amending spending on education.

Al Jazeera's Laurence Lee, reporting from London, said the speech in its entirety sounded like an "election manifesto".

"Johnson and his team have made it very, very clear that they don't want to be seen as only being interested in the delivery of Brexit on October 31," Lee said.

"They want to get a range of ideas and strategies in place ... not least because there could be a general election before Brexit is supposed to happen," he added.

Later on Wednesday, Johnson began his premiership with one of the biggest cull of senior government jobs in recent British history, changing all of the main ministers amid a spate of resignations.

As part of his new cabinet, he named Sajid Javid as finance minister and Priti Patel as interior minister. Ex-Brexit secretary Dominic Raab was named foreign minister, taking over from Johnson's Conservative Party leadership contest rival, Jeremy Hunt.

But leader of the main opposition Labour Party Jeremy Corbyn earlier on Wednesday accused Johnson of having "no mandate" to lead the UK and challenged him to hold a general election.

"We have had three years of bungled negotiations, and we now have the spectacle of a prime minister coming into office with no electoral mandate looking for a Brexit deal that has been ruled out by the European Union, or in the case of a no-deal, ruled out by the majority in this House and by anyone who understands the dangers to the British economy of a no-deal," Corbyn said in parliament.