Valentino’s honourary president and longtime friend, Giancarlo Giammetti, has shared a never-before-seen photo of the late Princess Diana aboard the fashion house's private yacht.

Taken in 1990, the photo shows Diana looking slim and stylish in a slouchy shirt tied over a printed bikini, topping off the look with a pair of large black sunglasses.

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The photo, which was taken on board the famous Valentino-owned T.M.Blue One yacht, also depicts Rosario Saxe-Coburg, who is married to Prince Kyril of Bulgaria.

"Summer is here TBT princess Diana on the TMblue 1990 #princessdiana #rosariosaxecoburg," Giancarlo captioned the throwback Instagram post.

Read more: 10 facts about Princess Diana

The Princess of Wales was a known yachting enthusiast, reportedly because it was one of the few places she could unwind in private.

Diana, mother to Prince William, 32, and Harry, 29, died in a car crash on August 31, 1997 in Paris.