Princess Diana's surprise Billy Joel performance was early sign of doomed marriage: author

King Charles was less than impressed by Princess Diana’s surprise for him.

In December 1985, the Princess of Wales shocked her husband – and the world – when she danced to Billy Joel’s "Uptown Girl" at the Royal Opera House in London. She was joined by her friend, acclaimed dancer Wayne Sleep. 

But the three-minute routine, which involved the princess sashaying in a slinky gown with no stockings, didn’t seem to move Prince Charles.

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Princess Diana is seen here dancing with Wayne Sleep at "The Friends of Covent Garden Gala" in London, circa 1985. (Shutterstock )

"She wanted Charles to approve of it," Diana’s ballet teacher, Anne Allan, told Fox News Digital. "When we met him afterward, he did smile and say, ‘Well done, darling.’ But [he] immediately went on to doing something else. And when you’ve come offstage and your adrenaline is pumping … you just want somebody to tell you that you’re doing all right."

Anne Allan's book, "Dancing with Diana," is available now.

Allan has written a new book, "Dancing with Diana: A Memoir," which details how she ended up working with and befriending the late British royal. The dance routine and its aftermath were immortalized in Season 4 of "The Crown" on Netflix.

"Perhaps he’s a man that doesn’t like surprises," Allan explained about Charles. "Firstly, not everybody does. Secondly, I’m not too sure what he thought about his wife wearing a slinky dress, dancing on stage showing her whole body, and showing her being. I’m not quite sure whether he thought it was the appropriate thing for Her Royal Highness the Princess of Wales."

Princess Diana wanted her performance to be a surprise for her husband, the former Prince Charles. (Shutterstock )

 "I’m not too sure what he thought about his wife wearing a slinky dress, dancing on stage showing her whole body, and showing her being."

— Anne Allan

Looking back, Allan believed that Diana was hoping to light a spark in her marriage.

"She loved Charles so much," said Allan. "She just wanted to bring romance into her life, because that’s what she thought it should be. And I know that they had many, many happy times together. . . .  She would arrange a nice night . . . and Charles occasionally would do that, too. But he was probably a bit more pragmatic as a person."

Anne Allan is seen here as a young ballerina. She would go on to give Princess Diana dance lessons. (Courtesy of Anne Allan)

"She did love romance," Allan shared. "I think she was much more able to see the possibility of how romance should be. I’m not sure if that was how Charles was. She loved holding hands, she loved having a cuddle. She was a very affectionate person. It meant a lot to her to have that in her life."

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Princess Diana managed to keep the performance top secret. (Shutterstock )

Diana turned to her love of dance as she navigated royal life – and a marriage that quickly crumbled in the public eye. According to the book, Allan gave the princess hundreds of secret one-hour ballet lessons from 1981 to 1989.

They were practicing in the studio one day when Diana told her she wanted to see herself dancing on stage. She also wanted the performance filmed.

Princess Diana and Prince Charles talking to ballet dancers at the Covent Garden Opera House after a performance of "Cinderella," circa 1987. (Princess Diana Archive/Hulton Archive/Getty Images)

"It was wonderfully innocent," said Allan. "Wayne came to set the number, because he is a wonderful choreographer. And immediately, the bond between them was just electric. They both had an absolute wicked sense of humor. Trying to get them to get down to work was quite something."

Princess Diana is seen here chatting with Wayne Sleep. The two used their height differences to their advantage.  (Ken Lennnox/Mirrorpix/Getty Images)

"They both knew Diana is beautifully tall, Wayne is short," said Allan. "He said, ‘Oh, let’s use the difference in height.’ He came up with this wonderful choreography, which is partly the story of the lyrics."

WATCH: PRINCESS DIANA'S BALLET TEACHER RECALLS SURPRISE 'UPTOWN GIRL' DANCE

Diana wanted to keep the routine a surprise for her husband. The trio worked together to keep the performance under wraps. In the end, Sleep announced that he would be doing a special performance in time for the holidays. No one suspected that Diana would be his dancing partner.

Allan said she had been "nervous" about a wardrobe malfunction.

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American singer-songwriter Billy Joel and model Christie Brinkley on the set of the music video for Joel's song "Uptown Girl," 1983.   (Vinnie Zuffante/Getty Images)

"The biggest worry for me was that she’d chosen to wear heeled shoes," said Allan. "I was so terrified we’d have a malfunction with either the wardrobe or the shoes. In between rehearsals, I ran home, came up with these elastics, and said, ‘Listen, I think we should wear them just in case that shoe falls off.’ She thought it was a great idea. 

Princess Diana and Anne Allan quickly bonded behind closed doors while training. (Courtesy of Anne Allan)

"I put pancake [makeup] on them so they would match the tone of the tights. In the end, she didn’t wear tights. . . . I felt it would be all right because I didn’t think for one minute there would be photographers there. And lo and behold . . . we got the photographs the next day. All I could see was these elastics on her shoes."

"[But] it was her idea, her design, her inventiveness of wanting to show herself dancing," Allan added.

Anne Allan made sure Princess Diana was wearing elastics on her shoes to avoid a stage mishap. (Shutterstock)

In the book, Allan wrote that she "sensed disapproval" from Charles after the performance.

"She just thought it would be great fun not telling Charles," said Allan. "She would just appear, and it would be hugely entertaining, and everybody would maybe have a nice time."

As the women grew closer, Allan learned how much Diana wanted to make her husband happy.

The former Prince Charles (now King Charles) seemingly disapproved of his wife's performance. (Tim Graham Photo Library via Getty Images)

"More than anything, she wanted to support her husband," said Allan. "That’s what mattered to her. She loved him dearly, and she just wanted him to shine. . . . When articles became more and more about her, she just said, ‘I do wish they would spend more time photographing Charles than I. I don’t want to make things difficult.’"

"It was important to her that she was doing her job well," Allan shared. "That mattered to her."

As a friend, Diana asked Allan for advice. But what came out of the royal’s mouth stunned the teacher.

Princess Diana would later reveal to Anne Allan that Prince Charles was seeing his ex. (Tim Graham Photo Library via Getty Images)

"I know he is seeing Camilla again," said Diana about her husband’s mistress, as quoted in the book. "Why does he not love me? I really don’t understand. 

Princess Diana confided in Anne Allan about her hopes and worries. (Courtesy of Anne Allan)

"I have tried everything, tried to conform to his wishes even though I don’t always agree. There’s no affection between us, and I am always on my own. I just want to be loved. I can’t keep going on like this. They are really expecting me to just say nothing and keep going. How do I do that?"

Diana would also tell Allan: "We manage to put on a good official front, but it’s getting more and more difficult, as I find it very hard not to show how I feel if I am upset. Charles seems to want us to lead separate lives. 

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Anne Allan said that Princess Diana wanted desperately to please her husband. (Jayne Fincher/Princess Diana Archive/Getty Images)

"Anne, he runs off to Camilla whenever he can. It’s not at all what I want. I’d like the marriage to work, but it just isn’t for now. Do I just put up with it, hoping he will change?"

Diana would later publicly describe Camilla Parker Bowles, who carried out a long-term affair with Charles, as the third person in their marriage.

Princess Diana famously described Camilla as the third person in her marriage. (PA Images via Getty Images)

When Diana learned from a friend that journalist Andrew Morton was writing a book about her, she decided it was time to tell her story. The pair used mutual pal Dr. James Colthurst as a go-between as Morton wrote his 1992 book, "Diana: Her True Story."

Princess Diana, nicknamed "Shy Di" for her quiet demeanor, was determined to have her voice heard. (Getty Images)

Allan claimed that Morton had reached out to her at the time for an interview. She passed. Still, she understood why Diana had chosen to get deeply personal.

"I thought that she must have been at the brink of things when she spoke to Andrew Morton," Allan explained. "I think perhaps she needed at that point to tell her side of the story. By then, things had become quite difficult. 

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Prince Charles and Princess Diana's divorce was finalized in 1996. (Jayne Fincher/Princess Diana Archive/Getty Images)

"But I will also say that when I left her in 1989, I was leaving a very strong woman. She had found her voice and was going to go forward being the Princess of Wales. . . . She wanted to express what she felt. [But] I think it was a long process for her to do that."

"She was just 20 when she joined the royal family," said Allan. "In the end, she was almost 30 with two children. She came into herself as a woman, and she realized that there were things that mattered to her. . . . Therefore, as she grew in confidence, she realized she could have a voice. She could have something to say."

Prince Charles and Princess Diana's marriage famously broke down on the world's stage. (Jayne Fincher/Princess Diana Archive/Getty Images)

Charles and Diana announced their separation in December 1992. Camilla and her husband, Andrew Parker Bowles, divorced in 1995. It was shortly after Charles gave an explosive television interview admitting he had an affair with her. Charles and Diana’s divorce was finalized the following year.

Princess Diana died in 1997. She was 36. (Tim Graham Photo Library via Getty Images)

Diana died in 1997 from injuries she sustained in a car crash. She was 36.

"I want everyone to remember the beautiful person that she was," Allan reflected. "The genuineness, the authenticity, the compassion and warmth. Anyone who met her saw it instantly. 

Anne Allan said she wants people to remember Princess Diana for having an open heart. (John Stillwell - PA Images/PA Images via Getty Images)

"She had an open heart. I think she would want people to have an open heart, too. She was an incredible woman who faced many challenges in her life, but she always stepped out with a smile on her face. In the end, she just wanted to do good."

The Associated Press contributed to this report.

Stephanie Nolasco covers entertainment at Foxnews.com.