London: Princess Diana probably knew about Camilla -- whom she referred to as the "third person" in her marriage to Prince Charles -- before tying the knot with the Prince of Wales, a new letter suggests.

The handwritten letter shows that two weeks before the Prince of Wales proposed, Diana Spencer spoke of a mysterious "she" who was the reason that she planned to go to Australia the following month, 'The Daily Telegraph' reported.

"Thinking back to 'she'(!) of our conversations I think it's so right for me to go to Australia and not long, 2½ weeks will do both of us good," Diana wrote to Raine, her father's second wife, on Jan 18, 1981.

"I only hope that in the end we will all be able to smile," the late Princess of Wales said in her letter which is to be auctioned in London later this month.

Two weeks later, the Prince proposed over a private dinner at Buckingham Palace. The then Diana Spencer subsequently made the private visit to Australia with her mother and stepfather before the engagement was announced.

In an interview with Martin Bashir in 1995, the Princess said she had known her husband had renewed a relationship with the then Camilla Parker Bowles in 1986, just five years after their wedding. "There were three of us in this marriage, so it was a bit crowded," Diana had said.

Prince Charles married Camilla, now the Duchess of Cornwall, in 2005, eight years after Diana's death in a tragic car crash in Paris in 1997.