Alongside a montage of photos of the royal on duty, were the words: "Wishing The Duke of Gloucester a very Happy Birthday today! [birthday cake emoji]."
The Duke, who is also known as Prince Richard, is the second son of the former Duke and Duchess of Gloucester, Prince Henry and Princess Alice. At the time of his birth, he was fifth in line to the British throne but is now 31st.
He is the first cousin of the late Queen Elizabeth II and the youngest grandchild of King George V and Queen Mary. He is a full-time working member of the royal family and attends and extensive slew of events both in support of The King and those which support his own charities and interests.
He is a doting father to three children whom he shares with his wife, Birgitte van Deurs. Their firstborn Alexander Windsor arrived on October 24, 1974, with daughter Lady Davina being born on November 19, 1977 and Lady Rose - who was born on March 1, 1980.
Him and his wife, who currently live in Kensington Palace, married in July 1972. The touching message from his royal family members came whilst more of The Firm arrive at Balmoral for the end of the summer holidays, a tradition that was started when Princess Victoria acquired the decadent estate.
So far, The King and Queen have received Princess Eugenie, with her sweet children, August, two, and baby Ernest in tow. Princess Beatrice has also made an appearance with her husband Edoardo Mapelli Mozzi and their family.
The jaw-dropping highland residence, which was a favourite of the late Queen Elizabeth II, spans around 50,000 acres, boasting an impeccable landscape of mountains, rivers, lochs, woodland, and gardens.
Whilst Prince William and Princess Kate are expected to attend the annual break, they are yet to arrive, although when they do, it is expected to be in the most down-to-earth way.
This time last year, the Prince and Princess of Wales flew economy to the Scottish destination. The Prince travelled separately with his eldest son Prince George, 11, whilst Kate travelled with Princess Charlotte, eight and Prince Louis, four.
The royal trio were spotted by one fan on TikTok who shared a video of them alighting their plane. She revealed: "[Kate] got up halfway through the flight to get her daughter's iPad. And yes she was stunning, even gave me a smile while looking back at the row behind her to talk to her son."