New leader of Britain’s Conservative Party Rishi Sunak waves outside the party's headquarters in London. Reuters

Former Treasury chief Rishi Sunak will become Britain’s youngest prime minister in modern times after he won the race to lead the Conservative Party, tasked with steering a deeply divided country through an economic downturn set to leave millions of people poorer.

One of the wealthiest politicians in Westminster, Sunak, 42, will become the country’s first leader of colour — and its third prime minister in less than two months - as he takes over during one of the most turbulent eras in British political history.

He replaces Liz Truss, who only lasted 44 days before she resigned, needing to restore stability to a country reeling from years of political and economic turmoil, and seeking to lead a party that has fractured along ideological lines.

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He told his lawmakers in parliament on Monday that they faced an “existential crisis” and must “unite or die.”

He told the country it faced a “profound economic challenge.”

Rishi Sunak (right) speaks during a parliament session as Boris Johnson looks on. File photo

“We now need stability and unity, and I will make it my utmost priority to bring our party and our country together,” said Sunak, who at 42 is Britain’s youngest prime minister for 200 years.

Sunak has ruled out holding a national election, according to lawmakers present at a speech given by him on Monday.

Rishi Sunak will be appointed Britain’s prime minister on Tuesday after outgoing leader Liz Truss submits her resignation to King Charles III, Downing Street said.

Truss will hold a final cabinet meeting before delivering a statement outside Number 10 around 10:15 am. Sunak will then be appointed by the king and make his own televised remarks around 11:35 am.

Rishi Sunak arrives for a meeting. File photo

Liz Truss said on Monday that Sunak had her full support after he won a leadership race to become the country’s next leader.

“Congratulations @RishiSunak on being appointed as Leader of the Conservative Party and our next Prime Minister,” Truss said on Twitter. “You have my full support.”

He defeated centrist politician Penny Mordaunt, who failed to get enough backing from lawmakers to enter the ballot, while his rival, the former prime minister

Former prime minister Boris Johnson, withdrew from the contest saying he could no longer unite the party.

Penny Mordaunt, who lost out to Sunak, said his election was an “historic one and shows, once again, the diversity and talent of our party,” she said. “Rishi has my full support.”