Prince Harry was said to be frustrated when his brother, Prince William, and sister-in-law Kate Middleton, allegedly did not "roll out the red carpet" for his wife, Meghan Markle.

Katie Nicholl, Vanity Fair’s royal correspondent, has written a new book titled, "The New Royals: Queen Elizabeth’s Legacy and the Future of the Crown." In it, she details the Duke of Sussex’s rocky relationship with the Prince of Wales, who is now heir to the throne.

Nicholl told Fox News Digital that after the Duke and Duchess of Sussex said "I do" in 2018, Harry was hoping that the Prince and Princess of Wales would show his new bride the ropes in successfully navigating royal life. However, the former American actress was allegedly left on her own.

Prince Harry and Meghan Markle's engagement

Prince Harry and actress Meghan Markle pose for photos as they announce their engagement at The Sunken Gardens at Kensington Palace on Nov. 27, 2017, in London. (Chris Jackson/Chris Jackson/Getty Images)

"It was certainly Harry’s feeling that William and Kate hadn’t rolled out the red carpet as much as they might have done," Nicholl said. "You know, there wasn’t this sort of instant welcoming of Meghan into the royal fold. I say in the book that William was the first to meet Meghan. And actually, he was really thrilled that Harry had met someone who he’d really fallen for. Because remember, at this time, Harry was very much on the shelf, a single man. Many of his friends were, by this point, married and having children. And Harry was genuinely worried he was going to be a bachelor forever. And so, he was overjoyed when Meghan came into his life, and he fell in love with this woman."

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"I think everyone in the family was thrilled about it," she added.

Harry, 38, first met the former "Suits" star, 41, on a blind date set up by a mutual friend. According to reports, the relationship went from "zero to 60." After rumors insisted that William was not happy with Harry’s decision to go public with Meghan, he disputed the claims, noting that he was supportive of his younger brother. However, Nicholl claimed that William was concerned about how the whirlwind romance was moving at lightning speed. In comparison, William and Kate, now both 40, dated for nearly a decade before getting engaged. Nicholl noted that the lengthy courtship was essential in helping Kate understand what royal life was like.

Prince Harry Kate Middleton

Author Katie Nicholl claimed that it was important for Prince Harry, left, to receive approval from Prince William and Kate Middleton, right, "who'd become a sister to him." (Victoria Jones - WPA Pool/Getty Images)

"I think there were concerns, certainly concerns from William’s part, that things had moved very quickly," she said. "That was really why William … cautioned Harry and said, ‘Just take your time. There’s no hurry in all of this.’ From Harry’s point of view, that basically wasn’t his brother, who he’d always been so close to, giving him his full seal of approval. And Harry wanted that. He wanted the seal of approval from both his brother and from Kate who’d … become a sister to him."

It had been alleged that William’s concerns, which were meant as "brotherly advice," led to a fallout between the princes. Harry, who is "hugely protective" of Markle, interpreted it as his brother not being behind the union.

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In late 2017, Harry proposed, and the couple tied the knot in May 2018. Rumors of a rift persisted when the joint office of the Sussexes and the Cambridges dissolved. Then in 2020, the Duke and Duchess of Sussex announced they were stepping back as senior members of the royal family. After moving to California, the couple gave an explosive interview to Oprah Winfrey in 2021. During the televised sit-down, Harry confirmed that he and his brother had been growing apart. While he noted that their relationship was "space at the moment," he added that "time heals all things, hopefully."

Oprah interviews Prince Harry, Meghan Markle

Oprah Winfrey interviewed Prince Harry and Meghan Markle for a special in 2021 where they revealed their struggles with royal life. (Harpo Productions/Joe Pugliese via Getty Images)

Nicholl claimed that things between William and Harry have never been the same.

"The relationship is not what it once was," she said. "This was sort of an unbreakable bond between the brothers. And there isn’t that closeness anymore. A lot has happened. I think there’s a lot of anger, hurt and resentment on both sides. I think many people had hoped that the queen’s death [in September] would be an opportunity for the brothers to come together, which of course they did."

"We saw them on that walkabout in Windsor," she continued. "We saw them together, walking side by side behind the coffin, which was so evocative because, of course, I think everyone that watched that couldn’t help but think back to the funeral of their mother. But ultimately, this wasn’t the moment for a reconciliation. And my understanding was that even though there was time when they could have seen each other during that period between the queen’s death and the funeral, it didn’t happen."

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Prince Harry and Prince William at the Queen's vigil

Prince Harry, behind, and Prince William reunited in September after Queen Elizabeth II's death. (Aaron Chown)

According to Nicholl, William "struggled" with Harry’s departure from the royal family. The brothers grew up together, supported each other after their mother’s untimely death and worked side by side as they began their royal duties. They were seemingly bonded for life. However, with Harry stepping away, not only was William losing his beloved sibling, he was allegedly concerned by how the extra workload in supporting the monarchy would impact his family.

"The impact of Harry leaving [is] possibly one of the reasons why William has struggled so much with this," Nicholl said. "Of course, it did have an immediate impact on William, his wife, Kate, and their young children. There was an increased workload almost overnight, which as a couple, they were very happy to undertake. But of course, it shifted the spotlight onto them and their young family in a way that wouldn’t necessarily have happened so soon. I think William always believed that Harry was going to be his wingman. He always believed that was the plan from day one … Harry would be there to support him."

Despite the alleged royal rift, Nicholl said sources have told her they are hopeful that William and Harry will reconcile when the time is right.

Prince Harry Prince William

Katie Nicholl said she is hopeful Prince William, right, and Prince Harry will eventually work out their differences. (Getty)

"Well, I’m always an optimist," she said. "I hope that they’re able to reconcile. I spoke to someone for the book who knows both brothers very well, who has worked with them for a good 10 years. And he told me that he believes there will be a reconciliation at some point in time."

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"I think it’s going to have to take forgiveness on both sides and a will to move forward, both for William and Harry, if this is going to show any signs of progressing," she said. "But right now, Harry has his own agenda. And William, he still finds it very difficult to accept what his brother did. And it’s not just in leaving the royal family but in how he left the royal family. The things that were subsequently said in interviews have been not just hurtful but very damaging as well for the royal family."

The Duke and Duchess of Sussex’s departure from royal duties began in 2020 over what they described as the British media’s intrusions and racist attitudes toward Markle. The family now lives in the coastal city of Montecito, California, with their two children.

The Associated Press contributed to this report.