Prime Minister Rishi Sunak and his wife Akshata Murty have attended a service with the King and Queen at Crathie Church near Balmoral.
Sunak and his wife, who are Hindus, were spending the weekend on the Royal estate in Scotland.
They arrived on Saturday and were driven to the 19th-century parish church overlooking the River Dee the following morning.
Also at the church were the King's sister, the Princess Royal and her husband, Vice Admiral Sir Tim Laurence.
The King was wearing a red and green tartan kilt with bright red knee-high socks. He showed off a new King Charles III tartan the previous day at the Braemar Gathering, which the Royals had been attending.
The late Queen Elizabeth spent almost every summer at Balmoral Castle. It was said to be one of her favourite places to stay, because she could live a relatively normal life - hosting family barbecues, driving by herself and doing the washing up.
She was a regular attendee at the local church, often slipping in to a back pew, wearing a headscarf to conceal her identity.
She was staying at the Castle, with the Princess Royal when she passed away. The anniversary of her death, and the King's accession to the throne is later this week.
Sunak's arrival at the Castle is part of an annual September tradition for sitting prime ministers, established during the late Queen's reign.
Parliament returned to the benches from summer recess on Monday.
She was staying at the Castle, with the Princess Royal when she passed away. The anniversary of her death, and the King's accession to the throne is later this week.
Mr Sunak's arrival at the Castle is part of an annual September tradition for sitting prime ministers, established during the late Queen's reign.
Parliament returned to the benches from summer recess on Monday.