The unearthed clip shows three-year-old William helping his mother re-do her makeup as he lovingly dabs her face with a powder pad, as the pair took part in a family photoshoot with Prince Charles and one-year-old Prince Harry in 1985.
Diana, wearing a pale blue polka dot dress, sweetly wrinkles her nose up at her eldest son before smiling at him, as William concentrates on reapplying his mum's makeup.
WATCH: Charles and Diana leave St Paul's Cathedral on their wedding day
One Instagram user commented on the clip: "So precious," while another said: "Beautiful so cute," and many others reacted with heart-shaped emojis.
The official photos from the shoot, taken by Tim Graham, show royal brothers William and Harry perched at a piano in Charles and Diana's Kensington Palace apartment.
In one frame, Diana gazes adoringly at her young sons, as they beam at their father Charles standing on the other side of the camera.
Diana with William and Harry at Kensington Palace in 1985
Charles and Diana tied the knot at St Paul's Cathedral on 29 July 1981 after a brief engagement. The couple welcomed their first child Prince William in 1982, followed by Prince Harry in 1984.
Their marriage broke down, and Charles and Diana separated in 1992, with their divorce finalised in 1996.
The Princess was tragically killed in a car accident in Paris in 1997, resulting in an outpouring of public grief in the UK and worldwide.
William and Harry have helped to continue Diana's charitable legacy by continuing to highlight issues such as homelessness, reducing the stigma around HIV and AIDS, and her campaign against landmines.
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