Rishi Sunak appointed Britain's Prime Minister by King Charles III

Rishi Sunak has become Britain's third prime minister this year and the first person of colour to lead the former imperial power, vowing to mend its stricken finances after Liz Truss lasted just 49 days.

Sunak spoke outside 10 Downing Street on Tuesday after his appointment by King Charles III, capping the latest extraordinary twist in UK politics following Boris Johnson's demise in July.

Sunak acknowledged mistakes by his predecessor Truss and promised to place “economic stability and confidence” at the heart of his agenda.

He said he would confront the “profound economic crisis” with compassion and lead a government of “integrity, professionalism and accountability.”

Departing Downing Street a little before, Truss wished Sunak "every success" and said she remained "more convinced than ever" that Britain needed to be "bold" in confronting the challenges it faced.

Sunak became the ruling Conservatives' new leader on Monday after triumphing over rival contender Penny Mordaunt, who failed to secure enough nominations from Tory MPs.

It had become a two-way fight after Johnson dramatically aborted a comeback attempt late Sunday, having failed to persuade Sunak to share power. He has maintained a deafening silence since.

Sunak, a Hindu, is the first British-Indian prime minister and, at 42, the youngest leader in more than two centuries.

US President Joe Biden called the choice "groundbreaking," and vowed to reach out to Sunak shortly.

Sunak took power in a morning audience with Charles – who anointed his first prime minister since ascending the throne following the death of his mother Queen Elizabeth II.

READ MORE: Rishi Sunak vows 'stability and unity' as UK's next prime minister

'Unite or die'

Britain's Conservative-supporting media hailed Sunak's appointment.

"The force is with you, Rishi," ran The Sun's headline, in a reference to Sunak's love of "Star Wars" films. The Daily Mail called it "a new dawn for Britain".

But the left-leaning Guardian highlighted Sunak's warning to Conservative MPs that the party must "unite or die".

Truss left office as the shortest-serving premier in history, after a calamitous tax-slashing budget sparked economic and political turmoil.

The 47-year-old announced her resignation last Thursday, admitting she could not deliver her mandate from Conservative members -- who had chosen her over Sunak in the summer to replace Johnson.

Sunak has now staged a stunning turnaround in political fortunes, and vowed to do the same for Britain as it confronts decades-high inflation, surging borrowing costs and imminent recession.

He also faces the uphill task of uniting a party riven with divisions and infighting.

Iain Duncan Smith, a former Conservative leader, said MPs now understood the "existential threat" facing the Tories, and that they needed to unite or accept being "out of power for a long time".

READ MORE: British PM candidate Sunak promises 'radical' income tax cut

'Difficult decisions'

After delivering the now all-too-familiar new leader's speech, Sunak started appointing his top team before facing his first session of "Prime Minister's Questions" in parliament on Wednesday.

Finance minister Jeremy Hunt – appointed by Truss just 11 days ago in a bid to salvage her premiership – could remain in the role after stabilising the markets.

He endorsed Sunak on Sunday, writing in the Telegraph that he was a leader "willing to make the choices necessary for our long-term prosperity".

After reversing almost all of Truss's various tax cuts, Hunt has warned "difficult decisions" loom over public spending.

Whoever heads the Treasury is set to unveil the government's much-anticipated fiscal plans on October 31.

Sunak must also decide whether to appoint to his cabinet senior MPs who did not support him, such as Mordaunt, in a bid to unify his fractured party.

One unlikely to get a seat around the table is former boss Johnson, who was driven out in July partly thanks to Sunak's resignation.

On Sunday he announced he would not go forward with his audacious leadership bid.

READ MORE: Britain’s political peril is unlikely to end with Sunak