A photograph of a teenage Princess Diana reclining across the lap of an unidentified male friend is going under the hammer.

Mystery surrounds the source of the image - which is scrawled with the words 'NOT TO BE PUBLISHED'.

The snap is thought to have been taken at a Swiss ski in 1979 or 1980, which would have made Diana 18 or 19-years-old. A bottle of whisky rests on the windowsill above her head.

princess diana

Back in the day: A fresh-faced Diana reclines on the lap of a young man

The image was discovered among the Daily Mirror's old photo archive and is marked with the date 26 February 1981 (believed to be the day it was bought) on the back - two days after Diana and Charles's engagement was officially announced by Buckingham Palace.

Owner Eric Caren, who bought the Mirror's photo library seven years ago, told The Daily Mail: "It's an intimate photo - I don't know the man in the picture and he could be a friend or a boyfriend.

"This would certainly not be in the way the Royal Family would have Diana to have been presented."

The paper cites sources who speculate the man may be Diana's former boyfriend Rory Scott, or close friend James Colthurst.

Strangely, the Mirror's then-picture editor is baffled as to where the snap came from. He told The Guardian he would never have written across a picture in case of future publication and that it is not his handwriting.

The image is being auctioned through RR Auctions in Hampshire as is being marketed as a "salacious teenage image of the future princess."

A spokesman told AFP the image would typically sell for around $1,000 (£750), "but because it has that (not-to-be-published) marking on it, we expect it to go for much more."

The Princess, who was mother to Princes William and Harry, was 36-years-old when she was killed in a car crash in Paris in 1997.

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  • FAMILY PICTURE OF LADY DIANA SPENCER IN HER PRAM AT PARK HOUSE, SANDRINGHAM, NORFOLK. * Picture Circa 1963/1964

  • Lady Diana Spencer aged 2 years old taken at Park House, Sandrigham, Norfolk

  • Lady Diana Spencer ages 3 years old at Park House, Sandringham, Norfolk.

  • A family album picture of Lady Diana Spencer at Park House in Sandringham, Norfolk, taken when she was a toddler, circa 1965

  • Lady Diana Spencer poses barefoot with a croquet mallet whilst on holiday in 1970 at Itchenor Sussex.

  • Family album picture of Lady Diana Spencer with Souffle, a Shetland pony, at her mother's home in Scotland during the summer of 1974.

  • Family album picture of Lady Diana Spencer at Itchenor, West Sussex, England in 1970

  • Princess Diana arrives at the Tate Gallery for the centenary gala dinner.

  • Princess Diana with H.R.H The Queen Mother at Remembrance Sunday with Prince Edward and Princess Anne.

  • Princess Diana with William

  • Princess Diana in India 1992.

  • PRINCESS DIANA CRIES WHILE ON VISIT TO ROYAL NATIONAL ORTHOPAEDIC HOSPITAL AT STANMORE. 1/7/92

  • Lady Diana Spencer aged 19, smiles at waiting photographers as she leaves her London flat for a shopping trip to Knightsbridge.

  • Lady Diana being pursued by the London press near her London flat aged 19

  • Lady Diana Spencer, youngest daughter of Earl Spencer, at work at a kindergarten in St. George's Square, Pimlico, London, where she was a teacher.

  • Lady Diana Spencer being pursued by the media as she leaves her London flat