The Labour leader, 62, was joined by his wife, Victoria , as they arrived at the monarch's Scottish residence on Saturday.
According to the Court Circular, the Prime Minister had an audience with the King on Saturday evening, and on Sunday, he joined Charles and Camilla at Crathie Kirk for the weekly church service.
It was just an overnight stay for the Starmers, with the couple departing from the Castle on Sunday, as Parliament returned from recess on Monday.
Keir became Prime Minister following Labour's landslide victory in the UK General Election in July.
Prime ministers are traditionally invited to stay with the head of state at Balmoral during the summer, and in the past decades Harold Wilson, who is said to have "got on like a house on fire" with Queen Elizabeth II , would join members of the royal family for riverside picnics on the Balmoral estate.
Former Prime Minister and Conservative leader David Cameron said there was not much "chillaxing" at Balmoral, with the royals spending their time on outdoor pursuits, such as hiking, fishing and picnics.
Theresa May, who was PM from 2016 to 2019, also recalled her time at Balmoral as she paid tribute to Elizabeth II after her death in September 2022. Find out what she said in the clip below…
Former Prime Minister Tony Blair's wife, Cherie, also revealed that their youngest son Leo was conceived at Balmoral.
In her autobiography, she shared that she left her contraceptives behind out of worry they would be unpacked by royal servants.
Charles, who donned a kilt, became patron of the society earlier this year as it marked its 200th anniversary, with the late Queen and King George previous patrons.
The King and Queen have been joined by their families during their traditional summer break in Scotland, including the Prince and Princess of Wales and their children, and the Duke and Duchess of Edinburgh.
Charles, who will mark the second anniversary of his reign on Sunday 8 September, is expected to attend the annual Braemar Highland Games on Saturday.
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