Clarence House shared a sweet black and white snap of Charles, aged 11, looking smart in a suit as he stands proudly behind his grandmother, who is holding two-month-old Prince Andrew. Princess Anne, then aged nine and dressed in a striped collared dress, beams as she tickles her baby brother.
Meanwhile, one of the Queen's corgis looks on with interest while sitting at the Queen Mother's feet.
WATCH: Duchess of Cornwall shares a look inside Clarence House
The image was taken in the garden of Clarence House, which was the Queen Mother's home at the time.
The Instagram caption read: "On this day in 1900, Her Majesty Queen Elizabeth The Queen Mother was born. The Queen Mother, The Prince of Wales's Grandmother, lived at Clarence House for nearly 50 years. The silver vase with rosemary seen in the first photo was given to The Queen Mother as a gift on her 100th birthday."
The sweet childhood image of the Queen Mother with Charles, Anne and Andrew
The Queen Mother resided at Clarence House from 1953 until her death at the age of 101 in March 2002.
Prince Charles carried out extensive renovation work before he moved into Clarence House, but has kept many of the rooms just as they were when his grandmother lived there – including the Horse Corridor.
The Queen Mother resided at Clarence House from 1953 to 2002
Photos show the space is decorated with red patterned wallpaper and gold furnishings, including bright curtains and wall-mounted paintings.
It is thought that Charles has kept the interior the same as a sweet tribute to his late grandmother, who had a passion for horses.
The Prince paid a moving tribute to his "irreplaceable" grandmother after her death.
In a televised statement from Highgrove House, he said: "Somehow I never thought [her death] would come. She seemed gloriously unstoppable, and ever since I was a child I adored her.
"She was quite simply the most magical grandmother you could possibly have and I was utterly devoted to her."
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