The image was bought by the Daily Mirror in the early 80s, ahead of Diana's marriage to Prince Charles. However, the paper apparently decided not to risk upsetting Buckingham Palace by showing the future King's betrothed lounging around with a different young man - and a bottle of whisky to boot - and filed the 8 x 10 black and white snap.

The paper's archive was later sold and the picture made its way to RR Auctions in New Hampshire, which believes the photograph's handwritten, double-underlined instruction not to publish will see it exceed the average going rate of £750 when it goes under the hammer later this month.

Bobby Livingstone of RR Auctions told AFP: "It captures the moment when the engagement had been announced two days before and the press was going mad over Diana - but the Daily Mirror just wasn't going to publish this picture of her in a comfortable position with a man other than Prince Charles."

Words: Anna Pollitt. Pictures: RR Auctions/ Rex Features

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Previously unseen photo of teenage Diana for sale