That exact moment has only been captured by photographs and no film. The annual show, Friends of Covent Garden, tends to feature a special appearance from a celebrity guest. In this case, it was Diana - unbeknownst to Prince Charles.
WATCH: Princess Diana showcases impressive piano skills in unearthed video
The room fell silent when she walked across the stage with famed British dancer Wayne Sleep, who performed for some years with the Royal Ballet.
Diana and the ballet star got to know each other after she contacted him to coach her for the surprise performance. They forged a close bond over their shared sense of humour and a love of dance he says would still be a part of her life.
"Oh yes, definitely," Wayne exclusively tells HELLO!. "She'd probably be teaching me how to keep my shoulders down and would be looking on Google to see what I was up to and if I was being naughty."
Princess Diana and Wayne Sleep were good friends
The dancer and choreographer, who is also an expert in jazz, tap and contemporary dance, became Diana's teacher after she persuaded him to give her clandestine lessons so she could slip on stage during a Royal Ballet gala event and surprise her husband the Prince of Wales.
"I walked in and I looked up at this towering sort of goddess above me and I thought: 'This Isn't going to work; it'll be hysterically funny,'" said Wayne, who was nine inches shorter than the royal.
"Then I said: 'You know, I've been up very late last night. Do you mind if I sit down?' And she said: 'No, not at all, you naughty boy.' From that moment, we started giggling and laughing."
"We had the same schoolboy humour," he added. "I was her sort-of jester, in a way, but we did get on incredibly well. We had something that clicked. She had natural style. She could do the high kick, she could do a pirouette and she had lovely swinging hips.
They did a surprise performance at London's Royal Opera House in December 1985
"She had that thing called charisma. She was charismatic and even when she spoke with that very quiet voice, it was something that pulled you in and you just wanted to embrace her all the time."
Nobody was allowed in the studio for the pair's top-secret rehearsals and when Diana slipped out of the royal box to join Wayne on stage for their performance to Billy Joel’s Uptown Girl – Diana's choice of music – the audience was stunned.
"They gasped; they were frozen," explained Wayne. "It took a while for them to actually believe it was really her."
Asked how they managed to keep their rehearsals a secret, Wayne revealed: "She would meet me somewhere and then we would slip into another part of the house. I would rehearse in the studio right down here in Chiswick then we would rehearse at my studio. We'd sometimes rehearse up where she was - so nobody ever got to know.
"But mind you a lot of it was done on the phone. I was on tour with my own company - doing eight shows a week and she'd be off somewhere like Australia – not sure where she was going. She had a job to do and so we had to do a lot of it over the phone."
The dancer has reflected on his fond memories with the late Princess
More than 30 years later, 72-year-old Wayne, who danced with the Royal Ballet Company and has starred in West End productions including Cats as well as TV shows such as I'm A Celebrity... Get Me Out of Here!, Strictly Dancer Fever and Channel 4's Big Ballet, is still teaching dance via Goldster.
The new venture is a first-of-its-kind platform for the over-50s that offers live virtual classes designed to aid healthy ageing.
"I'd put on a bit of weight so Goldster is my way of getting back into the fitness regime as well as teaching other people," he said. "I thought: 'Let's do it together.'"
The star then added: "Princess Diana would be on Goldster with me if she was alive. She asked me to teach her to dance when she decided to take up dancing again."
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