The Labour leader had his meeting in Buckingham Palace after Rishi Sunak stepped down following the worst ever general election result for the Conservatives.
Sir Keir now takes over as the first Labour prime minister in 14 years - and the seventh Labour prime minister ever.
Following the meeting, the King's spokesman said the King requested that Sir Keir formed a new administration.
"Sir Keir accepted His Majesty's offer and kissed hands upon his appointment as prime minister and first lord of the Treasury," he said.
AzVision.az
More about: UK SirKeirStarmer