Prince Harry and Prince William had a very close relationship as kids. They grew up having each other's back, even after Kate Middleton's arrival in their lives.

These days, however, seemingly tell a different story. After Meghan Markle's arrival to the royal Fold, things appeared to have changed between the royal siblings.

Although no confirmations have been made, many are certain that there is an ongoing rift between Prince Charles' two sons. The Duke of Sussex even made comments a few months ago that they are now taking different paths as brothers.

While the British royals have remained silent about the matter, an upcoming book will reportedly expose the true nature of Prince William and Prince Harry's relationshipBattle of Brothers, authored by Robert Lacey, will explore the ins and outs of the royal siblings' ties to each other, according to the Daily Express.

The material titled, Battle of Brothers: William and Harry - the Friendship and the Feuds, is reportedly set for an October release. It will reportedly center on the lives of the Dukes of Cambridge and Sussex as the "heir" and the "spare" of the British Monarchy.

As stated, the upcoming book targets to put the royal sibling's relationship into a "historical context." It will address the "unique and complex" ties between Prince William and Prince Harry amid the family's apparent and "recent woes," the royal author said.

Aside from their complicated relationship, Battle of Brothers will also reportedly touch on Kate Middleton and Meghan Markle's impact on the royal-born Princes' lives. Accordingly, it will feature the recent controversy surrounding the Sussex couple's exit from the British Royal Family, as per claims.

Robert Lacey, who has also become a royal consultant for Netflix Originals, The Crown, has found some of his discoveries as "astonishing," according to People. As stated, the publication obtained the information from the royal author's official press release earlier this week.

Alongside him being "astonished," he also reportedly shared how he has been "moved to tears" by the discoveries he made in the search for truths about Prince William and Prince Harry's relationship as brothers. It has been "enthralling," as well as "painful," the author said in the release.

On another note, a source reportedly told the publication that royal siblings' ties are "forever changed." Following all the controversies that have happened these past few years, things will no longer return to "the way they were," the source added.