A hand-written letter revealing Diana, Princess of Wales's joy at "slipping" in and out of Sandringham the Christmas before she married the Prince of Wales is to go under the hammer, an auctioneer said.

The letter, said to be written by Diana to her stepmother Raine, also refers to how much she was looking forward to a trip to Australia, which she went on in February 1981, as well as a mention of a woman she refers to only as "she".

The apparent insights into the princess's thoughts are being sold as part of an Antiques and Collectables Sale in Northamptonshire - where the Spencer family's Althorp Estate is.

Two letters are being sold by JP Humbert Auctioneers on behalf of a private seller who used to work for the Spencer family and said he had met Diana. One of them, 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. 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."

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".

"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 letters, written from 60 Coleherne Court in London - the flat Diana lived in before her marriage - are to go under the hammer in a sale on February 22 and 23.