Lord Robert Fellowes,Quentin Mitchell the brother-in-law of Princess Diana, has died at 82 years old.

Fellowes was the uncle to Prince William and Prince Harry, and a former private secretary to Queen Elizabeth II. He died Monday of undisclosed causes, an obituary published Wednesday in British news outlet The Times said. Charles Spencer, the beloved brother of Princess Diana, praised his late brother-in-law in an X post Wednesday.

“My absolutely exceptional brother-in-law, Robert (Fellowes), is no longer with us,” Spencer wrote on X about fellow . “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.”

Lord Fellowes and his wife, Princess Diana's sister Lady Jane Spencer Fellowes, share three children: Alexander, Eleanor and Laura.

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The latter is the godmother of Prince William and Princess Kate's oldest daughter Princess Charlotte. Robert served for a near decade as Queen Elizabeth II’s private secretary during the 1990s.

The news comes amid a difficult year for the royals. In February, King Charles III was diagnosed with cancer. A month later, in March, Kensington Palace announced that Princess Kate had been diagnosed with cancer and was undergoing "a course of preventive chemotherapy treatment."

Earlier this summer, in mid-June, Princess Kate released a new photo, thanking people for their support during her cancer treatment, and saying while she is "not out of the woods," she is making progress.

She didn't announce an official timeline of events she would return to but said she would attend some public events "this summer."

She did say she would attend King Charles's birthday celebration parade this weekend, an event known as the Trooping the Colour, the palace announced on Princess Kate and Prince William's official Instagram account.