A letter written by Princess Diana as she turned 30 is going up for auction.

The correspondence - from the late Princess of Wales to her close friend Dudley Poplak to thank him for three drawings he created for her 30th birthday in July 1991 - will be auctioned off by David Lay Auctioneers of Penzance, Cornwall.

In the letter, on Kensington Palace notepaper, Diana wrote: ''I do wonder what the next 10 years will bring, the last 10 I have learnt a great deal in an interesting way!''

Explaining that she had opened the present of the drawings before her birthday, she said: ''Since when have I be able to keep a present for the right day & I'm afraid your three wonderful drawings were no exception (sic).''

She included a joke about her stepmother Raine Spencer, writing: ''I am deeply touched to be given something that means such a lot to our family & that Raine hasn't put under auction!''

The auction lot will also include a note from Diana's mother Frances Shand Kydd, written four months after her death in 1997.

Mimo Connell Lay, of David Lay Auctioneers, said: ''These two items are from a retired dealer who is selling some of his most treasured items that he has kept for years. The letter from Diana is quite short but the content is strong.

''The line where she contemplates the past decade during which she got married, became a mother and her marriage broke down, and what the future holds for her is very poignant.''