Boris Johnson has given key cabinet roles to leading Brexiteers after becoming the UK's new prime minister.
Dominic Raab and Priti Patel return to government as foreign secretary and home secretary respectively.
Sajid Javid has been named as the new chancellor as more than half of Theresa May's old cabinet, including leadership rival Jeremy Hunt, quit or were sacked.
Earlier, Mr Johnson said the Brexit "doomsters and gloomsters" were wrong and the UK would leave on 31 October.
Speaking outside No 10, he said the UK would meet that deadline "no ifs, no buts", adding: "The buck stops with me."
Mr Johnson then turned his attention to a radical overhaul of the government, with 17 of Mrs May's former senior ministers being axed or stepping down.
Announcing his departure, Foreign Secretary Mr Hunt said he had been offered an alternative role but had turned it down.
1/4 I would have been honoured to carry on my work at the FCO but understand the need for a new PM to choose his team. BJ kindly offered me another role but after 9 yrs in Cabinet & over 300 cab mtgs now is the time to return 2 backbenches from where PM will have my full support— Jeremy Hunt (@Jeremy_Hunt) July 24, 2019
Defence Secretary Penny Mordaunt, a leading Brexiteer who is popular across the party, was the most surprising departure. She has been replaced by Ben Wallace, a former soldier and longstanding ally of Mr Johnson's.
Another prominent Brexiteer, International Trade Secretary Liam Fox, was also ousted, along with Business Secretary Greg Clark - a vocal opponent of a no-deal Brexit.
All three supported Mr Hunt in the Tory leadership contest.
Education Secretary Damian Hinds, Northern Ireland Secretary Karen Bradley, Immigration Minister Caroline Nokes, Culture Secretary Jeremy Wright and Communities Secretary James Brokenshire have also gone, along with Chris Grayling, whose record as Transport Secretary was much criticised.
Scottish Secretary David Mundell, who has left his position after four years, joked whether there would be "room" on the backbenches after all the dismissals.
Disappointed but not surprised to be leaving the Scotland Office after 9 years. Will, of course, support the new Government, but as I said to PM this afternoon I will also hold him to account on his commitments to the Union. Hope there’s still room on the backbenches! 🏴🇬🇧— David Mundell (@DavidMundellDCT) July 24, 2019
This comes on top of the earlier resignations of four leading ministers, including Chancellor Philip Hammond, Justice Secretary David Gauke and Cabinet Office minister David Lidington.
Conservative MP Nigel Evans described the changes as a "summer's day massacre".
The BBC's chief political correspondent Vicki Young said the sackings suggested Mr Johnson wasn't looking to build bridges across the party.
Instead, she said, he was focused above all else on assembling the team he thought would bring about the results he needed, even if that was controversial.
As the upheaval in government was happening, hundreds of people gathered outside the gates of Downing Street in protest against Mr Johnson's appointment.
Former Home Secretary Sajid Javid - a banker before entering politics - has been given the key role of chancellor, having thrown his weight behind Boris Johnson after being eliminated from the leadership race himself.
Priti Patel - who quit as international development secretary in 2017 after holding unauthorised meetings with Israeli officials - succeeds Mr Javid at the Home Office, where she said she would focus on keeping the UK country safe and fighting "the scourge of crime".
Dominic Raab is a former Brexit secretary, but quit over Mrs May's handling of the process.
He said he was "hugely humbled" by his appointment and said the UK needed to "bring finality" to Brexit so it could focus on the other big challenges.
Other figures involved in the Vote Leave referendum campaign have also been rewarded.
Michael Gove leaves behind his environment brief to become Chancellor of the Duchy of Lancaster, a more senior ministerial role but one without a specific portfolio.
Meanwhile, Jacob Rees-Mogg becomes leader of the House of Commons - his first government role.
Liz Truss moves from second in command at the Treasury to head the Department for International trade while Steve Barclay has been re-appointed as Brexit Secretary.
Health Secretary Matt Hancock and Work and Pensions Secretary Amber Rudd are among the few ministers who backed Remain who have kept their jobs. Ms Rudd also takes on the women and equalities brief.
Meanwhile, there is a speedy return to office for Gavin Williamson as education secretary.
He was sacked as defence secretary less than three months ago after No 10 concluded he was responsible for the leaking of unauthorised information from a National Security Council meeting - which he denied.
Mr Johnson's team has promised a record number of women in the cabinet. Nicky Morgan, Theresa Villiers and Andrea Leadsom have all returned to top jobs, taking on the culture, environment and business briefs respectively.
There are also promotions for Robert Buckland (justice) and Alok Sharma (international development) while former party chairman Grant Shapps, a key member of Boris Johnson's leadership campaign team, makes a comeback at transport.
Former Chief Whip Julian Smith is the new Northern Ireland Secretary, while Dumfries and Galloway MP Alister Jack, who was only elected to Parliament last year, is expected to become Scottish Secretary. Alun Cairns remains as Welsh Secretary.
Social care pledge
Earlier, in a 13-minute speech outside Downing Street, Mr Johnson listed a wide range of domestic ambitions, chiefly a promise to sort out care for the elderly "once and for all".
Reforms to the social care sector have eluded previous governments because of their cost and complexity.
"We will fix it once and for all with a clear plan we have prepared to give every older person the dignity and security they deserve," he insisted.
Mr Johnson also pledged to improve infrastructure, recruit 20,000 new police officers and "level up" school spending. He promised reforms to ensure the £20bn in extra funding earmarked for the NHS "really gets to the front line".
And he pledged to boost the UK's biotech and space science sectors, change the tax rules to provide incentives for investment, and do more to promote the welfare of animals.
Setting out his priorities for office, the former London mayor hit out at the "pessimists" who did not believe Brexit could be delivered and called for an end to three years of indecision.
"The people who bet against Britain are going to lose their shirts because we are going to restore trust in our democracy," he said.
"The time has come to act, to take decisions and change this country for the better."
He said he had "every confidence" the UK would leave the EU in 99 days time with a deal, but preparations for the "remote possibility" of a no-deal Brexit would be accelerated.
Mr Johnson vowed to bring all four nations of the United Kingdom - or what he described as the "awesome foursome" - together in the task of strengthening a post-Brexit country.
"Though I am today building a great team of men and women, I will take personal responsibility for the change I want to see," he concluded.
"Never mind the backstop, the buck stops here."
Labour's Sir Keir Starmer said Mr Johnson's speech was "all rhetoric". New Lib Dem leader Jo Swinson said she would welcome a cross-party push to find a solution on social care, but attacked Mr Johnson's "bluster and bravado" over Brexit.
Mr Johnson took over after Theresa May handed in her resignation to the Queen.
Earlier, as she relinquished power after three years, Mrs May said being prime minister had been "the greatest honour" and wished her successor well.
During his journey to Buckingham Palace, Mr Johnson's car was briefly held up by protesters from Greenpeace, who formed a human chain across The Mall.