Rishi Sunak twisted the knife on his predecessor Liz Truss after becoming the UK’s Prime Minister - the third in 50 days.

Sunak - who has become PM after meeting King Charles - pledged to “fix” the “mistakes” Truss made during her disastrous 49-day tenure as leader, the shortest in history.

He warned of “difficult decisions” to deal with the “profound economic crisis”, but added to struggling families: “I fully understand how hard things are”.

Speaking of Truss’ premiership, Sunak said “mistakes had been made”.

Rishi Sunak delivers a speech at 10 Downing Street after returning from Buckingham Palace where he was invited to form a government by Britain's King Charles III. Photo / Alastair Grant, AP

“Not born of ill will, or bad intentions,” he said. “Quite the opposite in fact, but mistakes nonetheless. And I have been elected as leader of my party and your Prime Minister in part to fix them.

“And that work begins immediately. I will place economic stability and confidence at the heart of this Government’s agenda.

“This will mean difficult decisions to come. But you saw me during Covid doing everything I could to protect people and businesses with schemes like furlough.

“There are always limits, more so now than ever. But I promise you this – I will bring that same compassion to the challenges we face today.”

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Sunak has become Britain’s first Asian PM and is the first practicing Hindu to become PM. At 42, Sunak is also the youngest person to lead the country since the Napoleonic wars.

Sunak was the sole candidate in the Conservative Party’s leadership contest after Boris Johnson pulled out and Penny Mordaunt was unable to secure enough backers.

New British Prime Minister Rishi Sunak arrives as Larry the cat watches on outside Downing Street in London. Photo / Alberto Pezzali, AP

Truss earlier gave a short speech outside 10 Downing Street before heading to Buckingham Palace to resign.

A defiant Truss said it was a “huge honour” to lead the country and wished every success to her successor.

“From my time as prime minister, I am more convinced than ever that we need to be bold,” Truss said, as her husband and two children watched.

“I wish Rishi Sunak every success for the country,” she added.

She finished her three-minute speech by saying: “I believe in the British people, and I know that brighter days lie ahead.”

Truss’ tenure lasted just 49 days - the shortest in UK history.

She handed in her resignation to the King, before Sunak arrived minutes later to “kiss hands” with Charles - the formal process of becoming PM.

King Charles III meets Prime Minister Liz Truss during their weekly audience at Buckingham Palace on October 12. Photo / Getty Images

The departing PM was forced to resign after her “mini-Budget” caused the pound to plunge in value, mortgage rates to soar and her party to sink to historic lows in the polls.

Sunak has immediately set about filling the posts in his government, aiming to put his stamp on the government while bringing in people from different wings of the Conservative Party.

He removed about a dozen members of Truss’ Cabinet, but kept several senior figures, including Foreign Secretary James Cleverly, Defence Secretary Ben Wallace and — crucially — Treasury chief Jeremy Hunt, who was appointed less than two weeks ago to steady the markets amid financial turmoil.

Home Secretary Suella Braverman, who resigned last week in a move that helped trigger Truss’ downfall, got her job back. A leading light of the Conservatives’ right wing, Braverman is charged with fulfilling a controversial, stalled plan to send some asylum seekers arriving in Britain on a one-way trip to Rwanda.

Sunak also brought back faces from the era of Truss’ predecessor, Boris Johnson, including Deputy Prime Minister Dominic Raab.

Sunak aims to assemble a Cabinet whose competence can erase memories of the missteps and U-turns of the past months. But the right-of-centre party’s divisions over immigration, relations with Europe and other big issues, remain deep. Allies of Truss and the scandal-plagued Johnson who have been sidelined or demoted from government can now nurture grievances from Parliament’s back benches.

“This is not a fresh start. It’s the same Conservative cabinet of chaos,” opposition Labour Party lawmaker Rosena Allin-Khan said on Twitter.

When he was Treasury chief, Sunak became popular with the public by handing out billions in support to shuttered businesses and laid-off workers during the Covid-19 pandemic.

But now he will have to oversee tax hikes and public spending cuts as he tries to bring inflation and government debt under control. A wave of strikes over pay by railway staff, telecoms workers, garbage collectors, lawyers and dockworkers is likely to spread.

Opponents already depict Sunak as out of touch with the concerns of ordinary people because of his privileged private school background, previous career as a hedge fund manager and vast wealth.

Much of Sunak’s fortune comes through his wife Akshata Murty, whose father is the billionaire founder of Indian IT firm Infosys. The couple are worth £730 million ($826m), according to the Sunday Times Rich List.

In April 2022, it emerged that Murty did not pay UK tax on her overseas income. The practice was legal — and Murty soon agreed to relinquish it — but it looked bad at a time when millions of Britons were struggling to make ends meet.

Rishi Sunak and his wife, Akshata Murty, in February at a reception at the British Museum in London. Photo / Getty Images

Sunak’s victory is a remarkable reversal of fortune just weeks after he lost to Truss in a Conservative election to replace Johnson.

Sunak was chosen as Conservative leader on Monday after becoming the only candidate to clear the nomination threshold of 100 lawmakers. Sunak defeated House of Commons Leader Penny Mordaunt — who keeps that job in his government — and Johnson, who failed to rally enough support for a comeback bid.

In addition to assembling a Cabinet, Sunak has to prepare for a Budget statement, scheduled to be delivered by Hunt on October 31, that will set out how the government plans to come up with billions of pounds to fill a fiscal hole created by soaring inflation and a sluggish economy — and exacerbated by Truss’ destabilising plans.

Truss announced her resignation last week and departed Tuesday after making a defiant public statement outside 10 Downing Street, seven weeks to the day after she was appointed prime minister.

Truss offered a defence of her low-tax vision and brief term in office, saying she was “more convinced than ever that we need to be bold and confront the problems we face.”

She leaves a Conservative Party trailing the left-of-centre Labour Party in opinion polls. Sunak has at most two years to turn its fortunes around. There does not need to be an election until the end of 2024, though public pressure to call an early poll is growing.

Jill Rutter, of the Institute for Government, said Sunak’s task was to show the Conservatives “are capable of governing in a fair way in the national interest”.

“If they continue to look like a party that is incapable of making decisions, incapable of making those decisions stick, then they will probably deserve to be punished by the electorate next time round,” she said.

Additional reporting from Associated Press.