Charles Spencer honoured his "exceptional brother-in-law," Lord Fellowes, who has passed away at the age of 82.
The Queen's former courtier was married to Lady Jane Fellowes, Charles' elder sister. According to The Times, Princess Diana's brother-in-law died of undisclosed causes on July 29.
Taking to his official X account, the Ninth Earl Spencer reshared his doting family member's obituary alongside a heartfelt message.
He penned: "My absolutely exceptional brother-in-law, Robert, is no longer with us. A total gentleman - in all the best meanings of that word - he was a man of humor, wisdom, & utter integrity. I'm deeply proud to have been his brother-in-law."
Robert married Lady Jane Spencer in 1978, and following their wedding, they welcomed three children: Laura Fellowes, 44; Alexander Robert Fellowes, 41; and Eleanor Ruth Fellowes, 38. The former royal courtier is also a godfather to Princess Charlotte.
While Lord Fellowes was linked to the Firm through marriage, his family also has a deep history with the British royal family.
Sir William 'Billy' Fellowes, Robert's father, was the land agent at Sandringham from 1936 to 1964, as well as a shooting companion of the Queen’s father, King George VI.
As for his mother, Jane, she was the daughter of Brigadier General Algernon Ferguson, a relative of Sarah Ferguson, who went on to become Duchess of York.
Robert successfully assisted the late Queen Elizabeth through her "annus horribilis" in 1992 when three of the Queen's children got divorced, and when a fire broke out at Windsor Castle.
Lord Fellowes was working for the Queen when Princess Diana, his sister-in-law, died, and he was part of the small team that advised the Queen to return to London from Balmoral.
He also, according to The Times, penned the first draft of the Queen's historic speech when she addressed the crowds as "your Queen, and as a grandmother" to the British public on the eve of Diana's funeral. The final version of this famous speech was written by Lord Fellowes, the Queen, and her private secretary, Sir Robin Janvrin.