Rishi Sunak on Tuesday became Britain's third prime minister this year and the first person of colour to lead the former imperial power, vowing to overcome economic crisis provoked by the "mistakes" of Liz Truss's calamitous 49-day tenure.

In his first order of business, Sunak retained Jeremy Hunt as chancellor of the exchequer, bidding to keep financial markets on side after Truss's budget plans shocked investors.

Sunak, a practising Hindu who at 42 is Britain's youngest leader since 1812, became the ruling Conservatives' new leader on Monday after a prior stint as chancellor himself.

Addressing the nation in Downing Street Tuesday shortly after his appointment by King Charles III, Sunak said the country faced "profound economic crisis".

"I will unite our country -- not with words, but with action," Sunak said, capping the latest extraordinary twist in UK politics following Boris Johnson's demise in July.

 

- 'Mistakes' -
 

Departing shortly before, Truss wished him "every success" -- and said she remained "more convinced than ever" that Britain needed to be "bold" in confronting the challenges it faced.

Sunak countered that though she was motivated by a well-intentioned desire to kick-start growth, her tax-cutting measures were "mistakes nonetheless".

"And I have been elected as leader of my party and your prime minister in part to fix them," he said.

"And that work begins immediately. I will place economic stability and confidence at the heart of this government's agenda," he added, helping to drive the pound more than one percent higher against the dollar Tuesday.

Sunak's appointment followed rival contender Penny Mordaunt failing to secure enough nominations from Tory MPs, and Johnson dramatically aborting a comeback attempt late on Sunday.

Breaking his silence, Johnson offered his "full and wholehearted support" to Sunak -- having privately blamed his ex-minister for toppling him in July.

Sunak in turn praised Johnson, and vowed to build on the election-winning promises that earned the Conservatives a big victory in 2019, despite their dismal standing in polling today against the opposition Labour party.

But Sunak also issued a coded reminder of the many scandals that brought Johnson down, vowing his own premiership would offer "integrity, professionalism and accountability at every level".

In other cabinet appointments, Sunak retained James Cleverly as foreign secretary, and brought close ally Dominic Raab back as deputy prime minister and justice secretary. He also retained Ben Wallace in the defence brief.

 

- 'Groundbreaking' -
 

In his Downing Street speech, Sunak pledged unstinting support for Ukraine even while warning of "difficult" budget choices ahead.

US President Joe Biden called the appointment of the first British-Indian prime minister "groundbreaking" and "pretty outstanding".

"Together, I look forward to enhancing our cooperation on issues critical to global security and prosperity, including continuing our strong support for Ukraine," Biden said in a tweet.

European leaders offered their own congratulations, while Irish premier Micheal Martin reminded Sunak of their "shared responsibility" to safeguard peace in Northern Ireland following tensions under Johnson and Truss.

Labour leader Keir Starmer praised Sunak on "making history as the first British-Asian PM".

But he added: "The Tories have crashed the economy, with low wages, high prices and a cost-of-living crisis. The public needs a fresh start and a say on Britain's future."

Sunak has rebuffed opposition calls for a snap general election after becoming the latest leader who lacks a direct mandate from the electorate, but he promised to govern on the basis of the 2019 manifesto.

Pollster Ipsos said that 62 percent of British voters want an election by the end of the year.

 

- 'Unite or die' -
 

Britain's Conservative-supporting media hailed the appointment of Sunak, a wealthy descendant of immigrants from India and East Africa.

"The force is with you, Rishi," ran The Sun's headline, playing on his love of "Star Wars" films.

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

Truss left office as the UK's shortest-serving premier in history, after her disastrous 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.

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".