Prince William’s concerns for his younger brother Harry may have caused a rift that still continues today, claims one royal expert.

British best-selling author Katie Nicholl participated in TLC’s documentary “Kate V. Meghan: Princesses at War?” that aims to examine whether there is any truth to the speculation of unrest following Meghan Markle becoming part of the British royal family.

PRINCE CHARLES REPORTEDLY ENDS FEUD BETWEEN MEGHAN MARKLE, KATE MIDDLETON

The show, which airs Tuesday, will specifically explore if there’s a possible rift between the former “Suits” star and her sister-in-law, Duchess of Cambridge Kate Middleton.

Some of the royal experts who have come forward for this special include Nicholl, as well as Lady Colin Campbell, royal biographer Ingrid Seward and Princess Diana’s former butler Paul Burrell, among others.

In the documentary, Nicholl claimed that since Markle, 37 and Harry, 34, announced their engagement in late 2017, William, 36, was growing increasingly worried for his brother and his whirlwind romance with the American actress.

“William was quite concerned that the relationship had moved so quickly,” claimed Nicholl. “And being close to Harry, you know, probably the only person close enough to say to Harry, ‘This seems to be moving quickly. Are you sure?’ And I think what was meant as well-intended brotherly advice just riled Harry.”

MEGHAN MARKLE, KATE MIDDLETON FEUD EXPLAINED

“Harry is hugely protective of Meghan,” continued Nicholl. “He saw that as criticism, he interpreted that as his brother not really being behind this marriage, this union. And I don’t think things have been quite right ever since.”

Lady Colin Campbell, a controversial royal commentator who has written several books on Diana alone, alleged Harry has changed since falling head over heels for Markle. She insisted that if there’s an ongoing feud between the brothers, it was most likely caused by Harry’s high-profile romance with the Hollywood star.

“Everything I hear is that Harry is completely beguiled by Meghan and completely enthralled to her,” claimed the 69-year-old, who alleges to have insider knowledge on the royal family.

“[Harry] has changed considerably,” alleged Campbell. “People do change when they get married.”

Seward, the editor-in-chief of Majesty magazine, has been covering the royal family since the 1980s and has written over 20 books on the subject. She agrees with Campbell’s analysis and said Harry was often spotted with William and Middleton, 37, before his relationship with Markle blossomed.

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“Before Harry met Meg, he was around Kensington Palace,” she explained. “He was eating with the Duke and Duchess of Cambridge. He was very much a part of their family. Then suddenly he meets Meghan and falls in love and he’s off. So he’s sort of suddenly out of their lives. And there may have been some resentment.”

Earlier this month, Princess Diana biographer Robert Jobson told Australia’s Sunrise he was convinced the princes’ father, Prince Charles, intervened on Christmas to end the reported spat between Markle and Middleton, whose relationship was under intense strain following a number of alleged standoffs.

“I’m sure at Christmas Prince Charles was knocking heads together and you can see that by those photographs of them all walking together in line,” said Jobson at the time.

However, Jobson admitted it will take time for the duchesses to act friendly naturally in each other’s company.

Commenting on the Christmas photo-op of the women walking to church together in Sandringham, Jobson said, “They looked like they had been pushed together in a line, to be honest.”

It’s unclear whether William and Harry have worked out their own differences.

In December 2018, Seward told Fox News that Markle and Harry’s plans to live in Frogmore Cottage over Kensington Palace comes as no surprise. She said Harry has always yearned to leave London for quieter, more private pastures.

“I don’t think Harry moving out has anything to do about a fallout with his brother,” said Seward. “Kensington Palace is a huge enclave. It’s like a village. And Harry always felt very restrictive living there. He has said as much. And I think he wanted to be away from Kensington Palace and the royals there. Maybe he wasn’t getting on with his brother as he used to, but families are like that. You don’t have to get on with your sibling all the time.”

Kensington Palace recently confirmed to Fox News that Markle and Harry will be spending their first Valentine's Day as a married couple apart. Instead, Harry is reporting for duty as Captain General Royal Marines in freezing Bardufoss, Norway to see how the Marines are training in the hostile environment 200 miles inside the Arctic Circle.

Markle is due to give birth to her first child with Harry sometime in the spring of this year.

"Kate V. Meghan: Princesses at War?" airs February 5 at 10 PM on TLC.