Boris Johnson took over from Theresa May as British prime minister on Wednesday, beginning his task to take the UK out of the EU by October 31.
Mr Johnson travelled to Buckingham Palace for an audience with Queen Elizabeth II in the afternoon before moving into Number 10 Downing Street, the prime minister’s residence.
"We're going to fulfil the repeated promises of parliament to the people and take the UK out of the EU by October 31," he said on the steps of No 10.
He pledged to do "a new deal, a better deal" with the European Union than the one rejected three times by the UK parliament.
However, he said it was "common sense" to prepare for a no-deal Brexit if Brussels was not willing to come to another agreement.
There were also promises on climate change action and animal welfare, which is seen as being a nod to his girlfriend Carrie Symonds, an environmental campaigner who watched his speech in the background.
Mr Johnson is expected to appoint pro-Brexit colleagues to senior positions in his cabinet, with sources close to the former foreign secretary saying his top team will reflect “modern Britain”.
He has already pledged to appoint at least one woman to lead either the Treasury, Foreign Office and Home Office.
But in a surprise move prominent Brexiter Penny Mordaunt, the current defence minister revealed she was leaving the government.
Jeremy Hunt, Mr Johnson's Conservative Party leadership rival, has resigned as foreign secretary in the midst of a worsening stand-off with Iran over captured tankers. Mr Hunt won around a third of votes cast in a ballot of Conservative Party members, but refused to become defence minister, a demotion from his post as foreign secretary. Former Brexit secretary Dominic Raab replaces Mr Hunt as foreign secretary.
International trade minister and prominent Brexiteer Liam Fox was sacked just hours after Mr Johnson's return from the palace.
His successor will be crucial to the UK planning future trade deals in a post-Brexit Britain.
Mr Johnson put hardline Brexiters in top positions, with former international development secretary Priti Patel appointed as home secretary.
Ms Patel was sacked from her ministerial position in 2017 after it was revealed she had conducted secret meetings with the Israeli government.
Gavin Williamson, another former minister fired by Ms May, was said to be unhappy after he was reportedly offered the position of housing minister.
Former home secretary Sajid Javid, who ran against Mr Johnson in the Conservative leadership contest, is Britain's new finance minister.
So far 17 Tory ministers either resigned or been sacked from their posts on Wednesday afternoon.
Mr Johnson promised to deliver Brexit during his acceptance speech in London on Tuesday, something current prime minister Ms May failed to do.
Theresa May stands down
Ms May took questions from parliamentarians in the House of Commons at midday in her last weekly prime minister's questions.
She was asked by opposition Labour MP Ruth Cadbury whether she was happy to be succeeded by a man who "demonised Muslims", does not support civil servants, and who was subordinate to US President Donald Trump.
"I am pleased to be handing over to someone who served in my cabinet," Mrs May responded.
Ms May, who will continue as an MP, ended her weekly exchange with opposition Labour leader Jeremy Corbyn by suggesting that he should follow her example and step down as party leader.
She later travelled to Buckingham Palace to give her resignation as prime minister to the Queen before the monarch invited Mr Johnson to form a government.
Before leaving for the palace, Ms May spoke outside Downing Street, flanked by her husband Philip, wishing her successor good luck.
During her speech, anti-Brexit activist Steve Bray could be heard shouting "stop Brexit" in the distance.
"I think not," Mrs May replied.
Sterling was down against the dollar on Wednesday morning at $1.2436, on track for its fourth straight day of losses and edging closer to $1.2382, the two-year low brushed last week, as the spectre of a no-deal Brexit was raised.
Climate protesters attempted to block the motorcade taking the new prime minister to see the Queen but were quickly dispersed by police.
A number of ministers who served in Ms May's cabinet resigned ahead of Mr Johnson's appointment, including the remain-supporting finance minister Philip Hammond and justice minister David Gauke.