Princess Diana told friends she "never would have gone through her divorce" if she'd realised how "desperate and ugly" it would be

In the notes, Diana told her pals she was left "on her knees" from the negotiations over the terms of her divorce and expressed her concerns the royal family had bugged the phone in her Kensington Palace apartment.

In one letter, written on 28 April 1996 and seen by the Daily Telegraph newspaper, 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."

And in a second, from the following month, she told Susie: "As I don't have a mobile it is difficult to discuss personal issues as my lines here are constantly recorded and passed on.

"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."

It was previously believed Diana had spent Christmas 1995 alone after Buckingham Palace announced she wouldn't be joining the royals, but in another letter, she thanked the couple for letting her spend the day with them while her sons Princess William and Harry were with their father at Sandringham.

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."

The Kassems - who met Diana for the first time in August 1995 during one of her regular visits to the Royal Brompton Hospital -decided to put the letters up for sale because "owning the documents is a great responsibility" which they don't want to pass on to their children and grandchildren.

The notes will be sold individually and are expected to fetch around £90,000, with the proceeds being donated to charities supported by the late princess.

Mimi Connell-Lay, of Lay's Auctioneers, who are selling the letters,

said: "Our clients are both aged in their 70s now. They have treasured these letters for over 25 years.

"They reflect the special and loving relationship they had with the most unique women that they had ever known."

The sale takes place on 16 February.