The image shows Diana, possibly in a ski chalet, smiling at the camera as she lies comfortably in the lap of a unidentified young man reading a book.
By the window stands a bottle of whisky, but more intriguing are the words ‘not to be published’ scrawled across the photo.
On the back, the photo is dated February 26, 1981 — two days after Buckingham Palace announced the engagement of Prince Charles and the commoner then known as Diana Spencer.
Bobby Livingstone of RR Auctions said the photo comes from the private Caren Archive, which acquired it seven years ago.
Livingstone said that the photo would typically sell for around $1,000, “but because it has that not-to-be-published marking on it, we expect it to go for much more”.
Internet bidding on the photo and others — including a rare autographed portrait of Albert Einstein will run from January 17 through 24.