Rumours have long circulated of a rift between Sarah Ferguson and Princess Diana before her death.
Sarah was once a close friend of the late princess and the pair had known each other since their early teens.
In the days when Sarah was married to Prince Andrew and Diana to Prince Charles, the two women were often seen both at official events and even on holiday together.
At the time of Diana's death in 1997, it was reported that the pair had not been on speaking terms for a year.
In her tell-all book, Finding Sarah, the Duchess claims she never knew the reason for the falling out but said "once Diana got something in her head it stuck there for a while".
But in a new interview, Sarah revealed she and the late princess made a touching promise to each other and that they were "so strong together".
Speaking to People, she explained: "We promised each other we would always be together - there was never any daylight between us. But everybody wanted that because we were so strong together. People want to break something so strong."
Sarah added that she and Diana were "best friends" from when she was 15 and the princess was 14. And she credits Diana, who she used to call "Duch", with teaching her about public life.
She said: "She was so brave. We used to have the most incredible time together."
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In the interview, the Duchess also revealed that if Diana were still here today, she would be proud of her sons Prince William and Prince Harry along with their wives Kate Middleton and Meghan Markle, who have their "own voice".
It's not the first time that Sarah has addressed rumours of tension between herself and Diana.
In an open letter published in Hello!, she wrote: "Women, in particular, are constantly pitted against and compared with each other in a way that reminds me of how people tried to portray Diana and me all the time as rivals, which is something neither of us ever felt."
While in 2018, she spoke of how she thought of Diana during her daughter Princess Eugenie's wedding.
She told MailOnline: "I thought of absent friends and family; of Diana - but she is with me all the time."
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