Sarah Ferguson betrayed Princess Diana with her selfish move

Royal sisters-in-law Princess Diana and Sarah Ferguson's relationship saw its ups and downs through out their lives. 

The two women were supportive of each other following their separation from their husbands. 

Now, Diana's former Butler Paul Burrell has weighed in on the two women's fallout.

There had been speculation that the Princess was unhappy with her former sister-in-law's previous remarks in her autobiography.

Writing in his new book, The Royal Insider: My life with the Queen, the King and Princess Diana, Paul reveals that the pair shared a good bond and would regularly have lunch together and confide in one another. However, they apparently distanced themselves from each other following the publication of the Duchess' autobiography, My Story, in 1996. 

According to Paul, Diana had no issue with Sarah decision to write autobiography, with one condition: she would not talk about her life, her children, or their relationship. 

But things took a nasty turn when Diana found Sarah had ignored her request, leaving her upset.

Paul claims that Diana essentially felt used by Sarah and stopped talking to her. At one point, they even 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 felt that 'now she is using her children to intervene. It's her last resort.'

However, following Diana's death in 1997, Sarah has always spoken fondly of her sister-in-law and best friend. 

Last year, she honoured the Princess with a sweet tribute on her 63rd birthday.