Queen Camilla and King Charles send message to senior member of the Royal Family

The photograph showed the Duchess dressed in a cream ensemble with a wide-brimmed hat and pearl necklace.

It was captured during her hosting of the Not Forgotten Association garden party at Buckingham Palace in May.

The birthday message featured a simple caption reading "Happy Birthday to The Duchess of Gloucester!" accompanied by a celebration emoji.

The Danish-born royal, who is married to the late Queen Elizabeth II's cousin Prince Richard, continues her extensive programme of royal duties.

Birgitte first encountered Prince Richard during the late 1960s whilst he was studying at Cambridge University.

The couple wed on 8 July 1972 at St Andrew's Church in Northamptonshire.

For her wedding day, the Danish bride chose a gown created by Norman Hartnell, one of the late Queen's preferred designers.

Rather than wearing a traditional tiara, she selected a floral headpiece to complete her bridal look.

Their union has now spanned more than five decades of dedicated royal service.

Just six weeks following their wedding, devastating circumstances altered the couple's future when Prince William of Gloucester died in a flying accident. This tragedy meant Prince Richard became heir to the Dukedom.

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When Prince Richard's father, Prince Henry, passed away in 1974, Richard and Birgitte assumed their current titles as Duke and Duchess of Gloucester.

The Gloucesters have three children together - Alexander, Davina and Rose.

Throughout these personal changes, Birgitte transformed into a senior working royal, undertaking hundreds of engagements annually in support of the Crown and numerous charitable organisations across Britain.

The Duchess made a notable appearance earlier this week at the Garter Day procession in Windsor, having been invested as a Royal Lady Companion to the Order of the Garter last year.

She also attended Royal Ascot on Wednesday, sporting an eye-catching floral hat designed by King's Foundation graduate Barnaby Horn.