Two letters from Diana to her stepmother are being auctioned by JP Humbert Auctioneers on Feb. 22 and Feb.23
In the handwritten letters, Diana is said to have said about the mystery woman, refering to her as "she" and how much she looked forward to her Australia trip, which she went on on Feb 1981, the Press Association reported.
The letters are being sold on behalf of a private seller who worked for the Spencer family.
One of the letters, which was written on January 18 1981, refers to a Christmas spent at Sandringham before the couple were married in July 1981.
"Dearest Raine, what a touching letter you wrote to me," it said. "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."
"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."
Another letter, written on headed notepaper just weeks before on December 28, 1980, thanks Raine for her gifts to Diana, saying everything is the "perfect colour" and she hopes to appear in a "vision of turquoise"
"Anyway, a million thanks for all your support and advice. I only hope that in the end we will all be able to smile. Lucky Daddy having you, much love from Diana."
"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."
"I am suffering from overeating and talking."
She thanks her stepmother again and wishes her the best for 1981.
Both the letters were from the address - 60 Coleherne Court in London - where Diana was living before her marriage.
The auction will take place at the company's saleroom in Towcester, Northamptonshire.