Watch: The Queen addresses the nation after death of Diana, Princess of Wales
Charles - the 9th Earl Spencer - took to Twitter to share a photo of the Spencer family flag that flies at their ancestral home Althorp, lowered to half-mast in tribute to the late Princess.
"My first family duty on 3 August. Never forgotten," he captioned the poignant image.
Charles shared a photo of the family flag lowered to half-mast
Charles later shared an image of flowers laid by members of the public at Diana's memorial, writing: "Lovely flowers from all sorts of people today, and recently, at Althorp House: thanks to all of them."
Charles and his family live at Althorp in Northamptonshire, which was built in 1508 and has been the Spencer family seat for 19 generations. It is also the final resting place of Princess Diana.
The 9th Earl Spencer also posted an image of flowers laid to mark the anniversary
She was laid to rest on an island in an ornamental lake known as The Oval within Althorp Park's Pleasure Garden. An ancient arboretum stands nearby, which contains trees planted by Prince William and Prince Harry, other members of her family and the Princess herself.
In a 2017 interview with People, Charles said his sister had "a genius for people and she could connect with anyone". He added: "She could make any person, whether they were the grandest or the most humble, totally at ease. It's an incredible gift."
Princess Diana is laid to rest on an island in an ornamental lake
It comes after Charles shared a poignant photo back in July, to mark what would have been Diana’s 59th birthday.
Rather than share a personal tribute to his late sister, the 56-year-old uploaded a powerful image taken in the grounds of his ancestral home. "Clouds above @althorphouse this evening," he wrote.
The late Princess pictured with her two boys
Many fans chose to remark on the significance of the image, with one writing: "Wherever she might be, she will always be remembered for her open arms for everybody everywhere. We do still miss you SO much Diana xxxxx." Another commented: "A message from your sister on her birthday."
Charles was the youngest child born to John Spencer, 8th Earl Spencer, and Frances Shand Kydd. They had five children in total: Lady Sarah McCorquodale, Jane Fellows, The Hon, John Spencer, who tragically died hours after his birth, Diana and Charles.
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