LONDON, Oct 24 (Reuters) - Rishi Sunak will become British prime minister on Monday after other candidates quit the race to lead the Conservative Party, leaving him with the task of steering a deeply divided country through an economic downturn set to leave millions of people poorer.

Sunak, one of the wealthiest politicians in Westminster, will be asked to form a government by King Charles, replacing Liz Truss, the outgoing leader who only lasted 44 days in the job.

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

"This decision is an historic one and shows, once again, the diversity and talent of our party. Rishi has my full support," Mordaunt said in a statement as she withdrew from the race just minutes before the winner was due to be announced.