She was referred to by Diana, the Princess of Wales as the "third person" in her marriage to the Prince of Wales.

© Press AssociationDiana, the Princess of Wales, spoke of a mysterious 'she' who was the reason she planned to travel to Australia the following month
Now a letter suggests that the Princess might have been worried about the Duchess of Cornwall before her wedding.

The letter shows that two weeks before the Prince proposed, she spoke of a mysterious "she" who was the reason she planned to travel to Australia the following month.

The handwritten letter was sent to Countess Spencer, known as Raine, her father's second wife, on Jan 18, 1981.

"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," the Princess wrote.

"I only hope that in the end we will all be able to smile."

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.

"I wasn't in a position to do anything about it," she said. "There were three of us in this marriage, so it was a bit crowded." The Prince married Camilla, now the Duchess of Cornwall, in 2005, eight years after the death of the Princess.

The letter is one of two to be sold on Feb 22 and 23 by JP Humbert Auctioneers at Towcester, Northants, near the Althorp estate, the Spencer family home.

They are being sold on behalf of a private seller who used to work for the Spencer family.

The first letter reads: "Dearest Raine, what a touching letter you wrote to me. It contained far too much praise which I'm sure I'm not due for.

"As you saw, I went to Sandringham and I arrived and left without being seen. I can't help but be pleased. Raine, you have been extremely kind and patient with all this and consequently I just wanted you to know how grateful I am."

She goes on to write of her proposed trip to Australia before ending: "A million thanks for all your support and advice. Lucky Daddy having you, much love from Diana."

The second, written on Dec 28, 1980, thanks Raine for her gifts.

"I spent a very happy Christmas at Althorp and thoroughly enjoyed the company, food and the very great trouble to make the day a success," she writes. "I am suffering from overeating and talking." She thanks her stepmother and wishes her the best for the year ahead.

Both letters were written from 60 Coleherne Court, London, the flat in which the Princess lived before her marriage.