The newly-unearthed letter describes Diana's isolation in the year before her separation
Diana described feeling "continuously misunderstood" and "extremely isolated" within the Royal household, particularly as she could see what was "coming to this country and this family".
The prescient letter was written just six months before author Andrew Morton's controversial book 'Diana: Her True Story' was published.
The biography sent shock waves through the country and the British monarchy as it revealed Diana's marriage to Charles was a sham and that they had both had extra-marital affairs.
Queen is pretty in pink as she joins Women's Institute meeting
Prince Charles hopes his Climate Change Ladybird book will lead to …
I do feel extremely isolated more than ever now, as I see what's coming
Although Morton claimed to have received the private details for his book from unattributed sources, he revealed after Diana's death in 1997 she had been the one who had fed him the information.
The Queen later famously described 1992, the year the book was published, as her 'annus horribilis'.
The hand-written letter which has now been unearthed was written by Diana to the late interior designer and close friend, Dudley Poplak.
The letter was written before Andrew Morton's biography revealed the royal marriage was a sham
Dated December 3, 1991, and written on Kensington Palace letterhead, she wrote: "Dearest Dudley, I did so enjoy our lunch today – it was special for many reasons and in particular our conversation towards the end of our delicious meal.
"I do feel extremely isolated more than ever now, as I see what's coming to this country and this family.
"During the last couple of years I have been through a self-development programme which has brought a deep understanding of those who suffer and those who need love and support.
Diana wrote the letter to her close friend Dudley Popak, an interior designer who died in 2005
"I feel that you believe in me, Dudley, and you've no idea what this means to me as I am continuously misunderstood by those around me."
"Please may we continue our conversation in January, when the pace of life isn't so fast and furious, as I'd care to ask for your advice and relay to you my inner thoughts."
She signed off: "Lots of love from, Diana x."
Mr Poplak died aged 74 in 2005 and the letter is being sold by a private collector.
The letter is being sold in a private auction and is believed to be worth £800
Chris Albury, from Dominic Winter Auctioneers of Cirencester, Glos, said: "The letter sent in December 1991 is poignant in that it was written only months before all the public falling out following the Morton biography.
"She must have been feeling even more isolated from the Windsors at a time when her marriage had fallen apart and she had just told her version of her Royal life to Morton, knowing that she was about to let a genie out of the bottle which would change her life for better or worse.
"This heartfelt letter offers a unique window into Diana's inner-torment at that time."
Princess Diana's charitable work
Thu, December 1, 2016
Princess Diana's biggest contribution to AIDS charity work was her public persona, she was one of the first notable people photographed touching and holding HIV/AIDS patients and many experts credit her with removing the stigma associated with AIDS. To mark World AIDS Day, Express Pictures looks back at the moments Princess Diana supported those suffering with HIV/AIDS.
Tim Graham/Getty Images
1 of 11
Diana, Princess of Wales, arrives at the Mortimer Market Centre, a sexual health and aids clinic in London on 1st December, 1994.
A second, shorter letter from Diana to Mr Poplak that is dated April 1991 in which she thanked him for sending her a book is also for sale at the auction.
And a typed letter dated April 2000 from Prince Charles to the same recipient in which the heir to the throne also thanks him for sending him a book and how he hopes to meet up with him soon is being sold.
Diana and Charles first met Mr Poplak in 1981 when he was called upon by her mother, Frances Shand Kydd, to work on Highgrove House – the Gloucestershire home of the Prince and Princess of Wales.
The royal couple's separation was announced by PM John Major a year after the letter was written
The royal couple's 'amicable separation' was announced to the nation in December 1992 – a year after Diana wrote the letter – by then Prime Minister John Major.
The letter has an estimate of £800. It is being sold on January 25.