After winning the head of the British government and the chairmanship of the Conservative Party , Rishi Sunak in his first speech after officially assuming the British Prime Minister's post pledged to fix the mistakes of former prime minister Liz Truss "immediately", but noted challenges that need hard choices.

Britain's King Charles III appointed new Conservative Party leader Rishi Sunak on Tuesday as the second prime minister of his reign, shortly after he accepted the resignation of Liz Truss.

After arriving at Buckingham Palace at 10.07 GMT, the 42-year-old former finance minister held his first meeting with the monarch.

I pledge to immediately start fixing Truss's mistakes," Sunak said, warning of the "difficult" choices ahead.

"I informed the king that I have accepted the assignment to head the government, I will work to unite our country with deeds, not words," he added.

"I will keep my promises and will strive to defend our borders, strengthen the economy and rise above political considerations,"he added.

Sunak said he would face "the deep economic crisis with compassion and lead an honest, professional and accountable government".

Sonak, 42, Britain's youngest leader in more than 200 years, must shore up an economy sliding into recession while also trying to unite a frustrated and divided party that is lagging behind the opposition in opinion polls.