'Fergie tried to guilt trip Diana with letter she pretended was by Eugenie after toxic fallout'

Royal sisters-in-law Princess Diana and Sarah Ferguson had a complex relationship. The two women became close during their time together in the family and leaned on each other following the breakdown of their respective marriages.

But in 1996 they fell out for good, with Sarah stating years later that she had no idea why Diana stopped speaking to her. There was much speculation that the Princess was unhappy about the way her former sister-in-law wrote about her in her autobiography, which had come out in November of that year.

One of the things to which Diana reportedly objected to was the claim that Diana gave Fergie several pairs of shoes, "and less happily her plantar warts".

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Now, Diana's former butler Paul Burrell has lifted the lid on the two women's fallout - and claims he knows the real reason the Princess cut ties with Sarah.

Writing in his new book, The Royal Insider: My Life with the Queen, the King and Princess Diana , Paul confirms that the pair formed a friendship at the start of 1982, and would share regular lunches and confidences.

By then Diana had been in the Royal Family for five years and so was able to offer Sarah valuable advice when it came to joining the Firm. But their friendship disintegrated following the publication of the Duchess' autobiography, My Story, in 1996.

According to Paul, Diana supported Sarah's decision to go public, with one proviso - she was not to talk about Diana, her boys, or their relationship. When the book was eventually published, Diana was "furious" to find Sarah had ignored her request.

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Refuting claims that they fell out over verrucas, Paul says the truth of the matter is that Diana felt 'used' by Sarah, and stopped speaking to her.

Instead they resorted to communicating through letters. "The last blow came when a letter arrived purportedly from Eugenie, Sarah's daughter, with the envelope written in a child's hand and addressed to HRH The Princess of Wales," Paul writes.

"Diana said to me, 'Look at this' as she held the letter head high. 'She is now using her children to intervene. It's her last resort.' The letter was actually from Fergie with a cruel postscript. It referred to a letter from Diana in which she had assured Fergie that nothing would ever appear from her about Fergie's life.

"Sarah went on to add a vitriolic swipe suggesting that 'Mr Kay' (the Daily Mail journalist) or even Mr Jephson (Diana's private secretary) might not agree with her. The relationship was terminated for good and they never spoke again. Diana's last words to Sarah were that when it came to honesty, perhaps she shouldn't resort to using her child to address envelopes to Diana and that Diana was happier than she had ever been."

The Mirror has reached out to Sarah Ferguson for comment.

Following Diana's untimely death in 1997, Sarah has spoken warmly of her "sister-in-law and best friend".

Last year she penned a tribute to the Princess on what would have been her 63rd birthday, alongside a photo of the pair together on the balcony of Buckingham palace.

Sarah wrote: "Happy birthday to my dear friend, Diana. You were a pillar of light and love. And what a legacy you have left behind. I will forever remember our laughter and the kindred, kind spirit I found in you. I am sure you are watching over us always. Rest in peace, my friend."

The Royal Insider: My Life with the Queen, the King and Princess Diana by Paul Burrell is out today, Thursday, September 11 (Sphere)