The photo was bought by the Daily Mirror two days after Diana's engagement to Prince Charles was announced in February 1981, and had 'Not to be Published' written in crayon on it.
The mystery man in the photo has been identified as aristocrat Adam Russell, the great-grandson of former Tory Prime Minister Stanley Baldwin. It is said Mr. Russell had a "galumphing" crush on Diana despite initially finding her 'a bit of a "sloaney giggler". When they were both injured while skiing he got a chance to know her and thought she was "bubbly and lacking in any self-confidence". It is understood nothing ever happened between them.
According to Diana's biographer Andrew Morton:
"Mr Russell went travelling for a year, and when he returned to the UK in 1980 and told a friend he liked Diana, he was told 'You've only got one rival, the Prince of Wales.' He tactfully decided not to pursue her, but the pair remained great friends."
Mr Russell is now a deer farmer in Dorset and married to gardening expert and TV presenter Ali Russell. It is thought the picture will fetch a significant amount because it was never published and is considered a rare find. It captures a moment in Diana's young life before her wedding and the difficult years that followed.