The King told Sir Keir Starmer he “must be exhausted” and “nearly on your knees” during a private audience with the Prime Minister on Friday.
Charles played a central role during the busy aftermath of the General Election, which left the Conservatives reeling following a crushing defeat by the Labour Party, which won by a landslide.
Sir Keir was welcomed at Buckingham Palace for a private audience with the monarch on his appointment as Prime Minister.
He swept into the palace just after noon in a chauffeur-driven car and was joined by his wife Lady Victoria Starmer, who works in occupational health for the NHS.
They were then greeted at the King’s Door by Sir Clive Alderton, principal private secretary to the King and Queen, and Commander William Thornton, Royal Navy, equerry to the King.
Sir Keir later had a private audience with the King who invited him to form a new administration.
In a video shared by Buckingham Palace, the PM could be seen entering the private audience room where shook hands with Charles.
The King quipped: “You must be exhausted and nearly on your knees.”
Sir Keir responded: “Not much sleep”.
Later in the meeting, the PM highlighted the “quick change around” since the results were first announced, to which the King remarked: “To say the least. And having to get to grips with everything straight away must be quite exhausting.”
Their historic meeting saw Sir Keir follow Rishi Sunak and Liz Truss to become the head of state’s third prime minister.
Buckingham Palace said: “The King received in audience the Right Honourable Sir Keir Starmer MP today and requested him to form 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.”
Lady Starmer joined the two men towards the end of their meeting, which lasted around 20 minutes, and at its conclusion, the Prime Minister and his wife headed to their new home in Downing Street.
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