Taking to Instagram, Fergie shared a picture of the duo standing side-by-side at the balcony of Buckingham Palace while watching the Battle of Britain Anniversary Parade on September 15, 1990.

Princess Diana and Ferguson were sisters-in-law through their respective marriages to King Charles and Prince Andrew.

Duchess Of York Remembers Princess Diana On Her Birthday

"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,” stated the Duchess.

In an interview with People in 2021, the Duchess reflected on her close relationship with the former Princess of Wales.

"We were best friends from when she was 14 and I was 15. I said to her, 'Oh, Andrew's really good-looking,' ” Fergie told the outlet and recalled the time Princess Diana introduced her to the Duke of York.

"Diana and I both had our own mental health issues, and she and I used to talk. She [Diana] said, 'Fergie, remember one thing: When you're at the top of the pedestal, it's so easy to fall off. And you're at the bottom. You just climb up," the Duchess further told the outlet.