Prince William, Kate Middleton learn from King Charles and Princess Diana's marriage mistakes: author

It’s been a year since Queen Elizabeth II passed away – and one author says the British monarchy is surviving with the help of its future king and queen consort.

Britain’s longest-reigning monarch passed away in 2022 at age 96. Her grandson, Prince William, is now heir to the throne.

"Prince William and his wife Catherine are very much beloved," author Andrew Morton told Fox News Digital. "Quite frankly, if Disney was to create a princess, a modern-day princess, I think Catherine would tick most of the boxes."

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The paperback version of Morton’s latest book, "The Queen: Her Life," was published on Sept. 5 with a new epilogue. It takes an in-depth look at the late queen’s life and legacy, one that is still impacting the monarchy.  The bestselling author, known for being Princess Diana’s biographer, has previously written books on other members of the British royal family, including Meghan Markle and Princess Margaret.

William, 41, will be in New York City on Monday and Tuesday for several engagements, including the second Earthshot Prize Innovation Summit.

"The great triumph… has been the fact that Kate has really come into her own as a Princess of Wales, as a future queen," Morton shared.

Over the years, William has allowed his wife to shine in the spotlight, as they remain committed to duty and raise awareness on causes they’re passionate about, Morton pointed out.

And the Prince of Wales has had two examples to live by. His grandfather Prince Philip spent more than seven decades supporting his wife. Since the queen’s coronation in 1953, the Duke of Edinburgh fulfilled more than 20,000 royal engagements to boost British interest at home and abroad and headed hundreds of charities. Britain’s longest-serving consort died in 2021 at age 99.

And then there was the turbulent marriage of William's parents, Princess Diana and the former Prince Charles. The former nursery schoolteacher turned glamorous icon brought lasting change to the royal family as she comforted AIDS patients, championed the vulnerable and led a campaign for a worldwide band on landmines. In her lifetime, she was a patron of over 100 charities.

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But her role as a royal rebel came at a price. Her marriage appeared doomed from the start and publicly disintegrated as she came out of her shell. Diana blamed Charles for continuing his liaison with his longtime mistress, Camilla Parker Bowles. She detailed her struggles behind palace doors in the 1992 book "Diana Her True Story," which was based on tapes the princess sent to Morton.

Charles and Diana’s divorce was finalized in 1996. And a year later, Diana died from injuries she sustained in a Paris car crash. She was 36.

WATCH: Prince William, Kate Middleton learn from Princess Diana's marriage

Morton said the lessons William learned from both marriages have stayed with him. The Prince and Princess of Wales have been married for 12 years.

"I think what he’s learned from his grandparents is stability and a sense of duty," Morton explained. "It’s not all about you, but it’s about others… And giving each other space in the marriage. I think that’s what the queen and Prince Philip did admirably."

"Prince Philip went his own way in terms of doing all kinds of charitable works," Morton shared. "He was the one who went out in the evening to various dinners to give dinner speeches. The queen was the person who stayed at home. So the queen’s marriage to Philip could be labeled a study in contrast."

"And I think it’s interesting that with Charles and Diana’s marriage, obviously he recognized that both parents loved the children, but didn’t love each other," Morton continued. "And obviously, he has had to cope, as his younger brother, with the death of his mother… I think the Prince and Princess of Wales have done a remarkable job in supporting each other."

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But the couple’s marriage hasn’t been without scrutiny. In the last few years, tabloids have published unfounded rumors of alleged infidelity. Morton pointed out that all royal couples, including the queen and Prince Philip, have faced the same gossip. The couple has followed the queen's mantra of "never complain, never explain."

"There’ll always be rumors about members of the royal family," he said. "It just comes with the territory."

For Morton, he admired the "quiet competition" the couple have in carving out their own identities in the public eye.

And it’s not only limited to royal duties. When the couple recently appeared on "The Good, The Bad, and The Rugby" podcast, they were compared to Monica and Chandler from "Friends" for their "super competitive" nature in sports.

"I’m really not that competitive," said Middleton, 41, with a laugh. "I don’t know where this is coming from."

William poked fun at his wife and appeared to give her a subtle wink, which was caught quickly by royal watchers.

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Middleton later agreed, saying that she and William haven’t "actually managed to finish a game of tennis" together.

"It becomes a mental challenge between the two of us," she said.

"Yeah, it’s who can out mental each other," said William.

Morton described the couple’s relationship as "an affectionate one," which they display when getting personal about their home life and passions.

"William doesn’t have the same chirpy charisma of [his brother] Prince Harry, but he’s focused on the job and is focused on his mission," he said.

Morton also praised Middleton’s transformation from a commoner to a well-loved princess.

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The couple first met at the University of St Andrews in Scotland. First friends and then housemates along with two other students, William and Middleton became romantically linked around 2004. Middleton graduated in 2005 with a degree in art history.

As the romance blossomed, William complained about press intrusion and Middleton’s lawyers asked newspaper editors to leave her alone. The couple briefly split in 2007. William later admitted that they were both young and trying to find their way.

The tabloids dubbed Middleton "Waity Katie" for her patience during their courtship. The couple married in 2011.

"I think [William] has done an admirable job… the word ‘training’ is wrong, but [it’s like] getting a thoroughbred ready for the big race," said Morton. "And it’s taken [Middleton] some years to become confident and competent in the role of princess, but I think now you are seeing her in her best years. She looks great. She has found her… footing as a royal in terms of charity work with the early age stuff."

"She’s very confident doing her own engagements," Morton continued. "She’s not leaning on William all the time. And I think that she’s genuinely come of age as a princess. And I say that because it’s not easy being a princess or a duchess for that matter. It takes a long time to get to understand the role. It’s more than just being a celebrity – there’s got to be something about you that transcends who are you."

All eyes are on the royals and what the future holds for them. King Charles III ascended to the throne upon the queen’s death. But Morton noted that the Prince and Princess of Wales have been supportive of the new king and queen as they prepare for their own destiny.

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"It’s funny, [the queen] is still on top of the popularity charts, so it’s as though she [hasn’t] left us," said Morton. "From that point of view, I think King Charles has done a very capable job. He’s done everything that’s been expected of him, as has Queen Camilla. They’ve largely ignored all the noise… and they’ve focused on the job of being a monarch and consort. And they have the right support."

The Associated Press contributed to this report.