Letters written by Princess Diana reveal anguish at 'desperate and ugly' divorce from Prince Charles

The late Princess of Wales confided in friends "she never would have gone through with her divorce" if she knew how "desperate and ugly" the process would be.

Previously unseen letters written by Princess Diana reveal her anguish at separating from Prince Charles, describing how negotiations over the terms of the divorce left her "on her knees".

Diana also shares her paranoia that her phone at Kensington Palace had been bugged by the royal family.

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The 32 letters from Princess Diana were written to her friends Susie and Tarek Kassem.

They have chosen to sell the collection at auction believing that the "ownership of these poignant documents is a responsibility" they "do not wish to pass on to their children or grandchildren".

The letters will be sold by Lay's Auctioneers in Cornwall on February 16 and are expected to reach close to $160,000 AUD.

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Some of the proceeds will go to charities Diana supported during her short life.

Diana met the Kassems at the Royal Brompton Hospital during her regular visits.

The letters being sold were written between 1995 and 1996.

In one of the most personal notes, dated April 28, 1996, Diana expressed her feelings about the breakdown of her marriage.

Diana and Charles separated in 1992 but their divorce was not made final until August 1996.

She had written to the couple to apologise for having to cancel a planned trip to see the opera Tosca.

Diana wrote: "I am having a very difficult time and pressure is serious and coming from all sides.

"It's too difficult sometimes to keep one's head up and today I am on my knees and just longing for this divorce to go through as the possible cost is tremendous."

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In another, sent after Christmas in 1995, Diana thanked the Kassems for inviting her to spend the day with them after Buckingham Palace had announced she would not be joining her sons Prince William and Prince Harry at Sandringham with the rest of the royal family.

She wrote: "I was so thrilled to be invited into a family occasion particularly as I was made to feel like one of the team".

In a letter dated May 20, 1996, Diana expressed her concerns that recordings of her private conversations were being passed on to the royal family, fearing her phone at Kensington Palace was bugged.

"As I don't have a mobile it is difficult to discuss personal issues as my lines here are constantly recorded and passed on," she wrote.

"If I'd known a year ago what I'd experience going through this divorce I never would have consented. It's desperate and ugly."

The auction house says the Kassems have chosen to keep some of the "more personal and confidential letters" written by Diana.

But those up for sale "illustrate Diana's immensely warm and loving disposition in a charming and delightful manner".

"Some letters do touch on the enormous stress she was experiencing during periods of very public heartbreak, yet her strength of character and her generous and witty disposition shine through," it adds.