Prior to her marriage to Prince Charles, the 1981 letter from Diana to her stepmother, Countess Raine Spencer, indicated she felt secure enough to make a lengthy trip to Australia following a conversation with someone referred to only as "she."
"Thinking back to 'she'(!) of our conversations I think it's so right for me to go to Australia and not long, 2 1/2 weeks will do both of us good," Diana wrote.
"I only hope that in the end we will all be able to smile," she added.
The Daily Telegraph said Saturday the handwritten letter was composed at Diana's London apartment just two weeks before Charles proposed to her. Diana later said she knew her husband had rekindled his relationship with Camilla after they had been married.
The newspaper said the intriguing letter was one of two she wrote to the countess in the early 1980s and will be auctioned off this month on behalf of a seller who used to work for Diana's family.
Diana died in a 1997 car crash in Paris and Charles married Bowles, now the duchess of Cornwall, in 2005.