Unseen photo of Princess Diana up for auction

This photo of a young Diana Spencer - soon before becoming Princess Diana - is to go under the hammer at an upcoming U.S. auction.

The candid snap shows the princess-to-be smiling for the camera alongside an unidentified young man.

The photo originally came from the library of Britain's Daily Mirror tabloid newspaper.

Scrawled onto the photograph itself the are the words "Not to be published"

On the back, the photo is dated February 26, 1981 -- two days after Buckingham Palace announced her engagement to Prince Charles.

The mother of the second in line to the throne, Prince William, died in a Paris car crash in August 1997, a year after her divorce from Charles.