Mr Sunak also paid tribute to outgoing prime minister Liz Truss for her "dedicated public service" in a brief speech at the Conservative Party headquarters in Westminster.The title of youngest ever prime minister belongs to William Pitt the Younger, who was just 24 when he took office in 1783.The former chancellor is taking over from Ms Truss, just two months after she beat him in the last leadership election.Mr Sunak won the leadership race after Commons leader Penny Mordaunt bowed out at the eleventh hour on Monday, having failed to get the 100 nominations from Tory MPs required by the 2pm deadline.Her decision came after Boris Johnson also withdrew from the contest on Sunday evening, despite claiming he had the backing of at least the 100 MPs required to make it on to the ballot."