The royals descended upon Windsor Castle on Wednesday for the King's annual pre-Christmas lunch.
Charles, 75, was joined by the Princess Royal and Vice Admiral Sir Tim Laurence, the Duke and Duchess of Edinburgh and Lady Louise Windsor, the Duke of York and Princess Eugenie and Jack Brooksbank.
While the gathering is usually attended by extended members of the King's family, there was another very special guest among the fold.
Penny Knatchbull, Countess Mountbatten of Burma, 70, was spotted among the guests arriving at Windsor Castle.
Countess Mountbatten was a close friend of the late Queen Elizabeth and the late Duke of Edinburgh.
Penny is the wife of Norton Knatchbull, 3rd Earl Mountbatten of Burma – who is the grandson of Prince Philip's beloved uncle the 1st Earl Mountbatten, and who was murdered by the IRA in 1979, making him a second cousin to King Charles.
The then Prince of Wales escorted Earl and Countess Mountbatten's daughter, Lady Alexandra Hooper, down the aisle on her wedding day in 2016.
Charles was also the Earl Mountbatten's best man when he married Penny at Romsey Abbey in 1979, and in return, Charles asked his friend to be godfather to his eldest son, Prince William.
Penny is a frequent guest at royal events, including the Royal Windsor Horse Show and shared Prince Philip's love of carriage driving.
She was one of only 30 mourners at the late Duke's funeral on 17 April 2021 at St George's Chapel and she attended the late Queen's funeral at Westminster Abbey in September 2022.
Elizabeth II and Philip spent their honeymoon at Earl Mountbatten's family seat, Broadlands in Hampshire in 1947.
Similarly, the newly-married Charles and Diana, Princess of Wales also stayed at Broadlands for part of their honeymoon in 1981.
LISTEN: A Right Royal Podcast Christmas Special