Britain's incoming prime minister Rishi Sunak appealed for unity at a time of crisis on Monday after he was crowned Conservative leader.
Mr Sunak will become prime minister on Tuesday morning after Liz Truss has chaired her final Cabinet meeting and visited the king at Buckingham Palace.
Mr Sunak, 42, will be Britain's first Hindu prime minister, its first of Asian descent, and its youngest for more than 200 years.
"We now need stability and unity and I will make it my utmost priority to bring our party and our country together," he said in his first public statement as leader.
"That is the only way we will overcome the challenges we face and build a better more prosperous future for our children and our grandchildren.
"I pledge that I will serve you with integrity and humility and I will work day in, day out to deliver for the British people."
Markets reacted positively as Mr Sunak took on the task of steering Britain through stormy economic waters. The blue-chip FTSE 100 rose 0.6 per cent.
He faces an array of problems including high inflation, rising energy and mortgage costs, nervous financial markets, the war in Ukraine and the continuing fallout from Brexit.
Tory MPs echoed Mr Sunak's call for unity, after weeks of party warfare led to Ms Truss being ousted after 45 days and the Conservatives slump to their worst poll ratings since the 1990s.
It capped a remarkable comeback for Mr Sunak after he resigned as chancellor of the exchequer in July, helping to force out Mr Johnson in the process, and was defeated by Ms Truss for the leadership last month.
MPs hugged and applauded him as he arrived at party headquarters as the newly crowned leader.
"Rishi will provide the calm, competent, pragmatic leadership our country needs at this deeply challenging time," said former prime minister Theresa May.
Alicia Kearns, the newly elected chairwoman of Parliament's foreign affairs committee, said: “It’s a return to grown-ups at the table, a return to fiscal responsibility and return to compassionate Conservativism, which are all good things."
Mr Sunak will be formally appointed after Liz Truss formally tenders her resignation to the king, who was returning to London from his Sandringham estate on Monday.
Born in Britain to a family of Indian heritage, Mr Sunak's appointment was welcomed by admirers as a historic moment.
"This decision is a historic one and shows, once again, the diversity and talent of our party. Rishi has my full support," Ms Mordaunt said as she conceded defeat. Ms Truss also offered her full support.
Sunder Katwala, the head of race relations think tank British Future, said Mr Sunak's appointment would not have been possible "even a decade or two ago".
"This will be a source of pride to many British Asians — including many who do not share Rishi Sunak’s Conservative politics," he said.
Mr Sunak was confirmed as leader after party rulemaker Sir Graham Brady said no other valid nominations had been received before a 2pm deadline.
He did not reveal whether Ms Mordaunt would have had the 100 nominations from MPs needed to force a ballot. But her allies said she had about 90, while Mr Sunak had almost 200. Mr Johnson withdrew on Sunday.
Sir Christopher Walker, whose incensed appearance after a House of Commons voting debacle last week summed up the Tory anger of the past month, said there was newfound unity on display as Mr Sunak gave a private address to MPs.
“If last Wednesday night was a low point, then that was a high point,” he said.
Senior backbench MP Crispin Blunt told The National that Mr Sunak had given “an awesome performance” that was “full of energy”.
Ms Mordaunt's withdrawal means the leadership question will not be put to the grass roots Tory membership.
Members chose Ms Truss over Mr Sunak in the summer contest to succeed Mr Johnson, but her leadership imploded in barely six weeks after a botched mini-budget.
The race to replace Ms Truss was compressed into a matter of days so that the new prime minister could quickly take charge of Britain's economic crisis.
Mr Sunak, a former chancellor, had touted his experience to MPs in guiding the economy through the coronavirus pandemic.
Supporters said his economic plans were vindicated after he warned against the tax cuts that backfired on Ms Truss, while critics described the wealthy Mr Sunak as out of touch.
In a second campaign that took place largely behind the scenes, he promised to show integrity and professionalism after the chaos of recent months.
Mr Johnson was forced out in July after a series of scandals wore down the patience of voters, MPs and ultimately his own ministers, including Mr Sunak.
The former prime minister claimed on Sunday he had the 100 supporters needed to advance, but conceded he would struggle to govern a party divided over his legacy.
The opposition Labour Party is calling for a general election after a second change of leadership since the Tories won a majority in 2019.
"Nobody voted for this," said Labour's deputy leader Angela Rayner.