The two women were friends before they both married members of the Royal Family. While Sarah was invited to Diana and Prince Charles’s wedding at St Paul’s Cathedral, she was not asked to the reception at Buckingham Palace.
The reason behind the snub is thought to have been royal protocol rather than Diana’s choice.
But Sarah – who was not in a relationship with Prince Andrew at the time – was “astonished” at being left off the guest list, royal expert Tom Quinn claimed.
He told Channel 5 documentary Fergie Vs Diana: Royal Wives at War: “I think she was astonished to discover she wasn’t being invited to the wedding breakfast.”
Psychologist Jo Hemmings said: “We know Fergie, who wrote an autobiography, felt really quite worthless and unfit as a person that she wasn’t invited.”
And Sarah’s friend Lizzy Cundy added: “I don’t know whether Diana realised how hurt actually Sarah was about this.”
Sarah was also devastated when she was turned down as Diana’s lady-in-waiting, according to Ms Cundy.
She said: “Sarah was told that she was unsuitable to be Diana’s lady-in-waiting, which was a huge blow to Sarah.
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Diana wed heir to the throne Charles on July 29 1981.
They went on to have Prince William and Prince Harry.
But they split in 1992 and divorced in 1996.
A year later Diana died in a tragic Paris car crash.
Sarah married Andrew at Westminster Abbey on July 23 1986.
The Duke and Duchess of York also parted ways in 1992 and finalised their split in 1996.
However, Sarah and Andrew have remained on good terms over the years sparking rumours of a reunion.
Sarah has previously described them as the “world’s happiest unmarried couple”.
