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

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

"I will unite our country — not with words, but with action," the former finance minister said, pledging also unstinting support for Ukraine even while warning of "difficult" budget choices ahead.

Sunak said a disastrous budget that felled Truss was motivated by a well-intentioned desire to kick-start growth, but its 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."

Departing Downing Street shortly 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.

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

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.