Prince William and Prince Harry were known for years for having a “once unbreakable” bond, but that has all changed over the past 18 months, which has seen the “devastating breakdown” of their relationship, according to a bombshell new book.

The rift between the royal brothers is explored in a new book by renowned royal biographer Robert Lacey, who said he was “astonished” and “sometimes moved to tears” by the revelations that he came upon about their relationship as he researched the book. Lacey explained that he came to the conclusion that the  “seeds of damage were sown” when Willam and Harry’s parents’ marriage fell apart in the early 1990s.

Daily Mail reported that the book will also take a look at how William and Harry’s relationship has been impacted by their wives, Kate Middleton and Meghan Markle, who have long been rumored to not get along.

“These two brothers – once inseparable and now separated by much more than mere distance – have been acting out the contradictions that go back into their childhoods and even before that: into their parents’ ill-fated marriage,” Lacey said. “We have seen conflicts between heir and spare in every recent generation of the royal family — but nothing so profound as this.”

He went on to say that he found the book, which is called “Battle Of Brothers,” to be both “painful and enthralling” at times to write.

“I have been astonished and sometimes moved to tears by the fresh details and insights I have discovered in researching this story of family conflict,” Lacey said. “‘It has been both enthralling and painful to trace this drama through the memories of close witnesses and some of the people most intimately involved.”

The synopsis of the book states that it will explore the “untold reality” of William and Harry’s relationship.

“Much reported on but little understood, Diana’s boys have lived under constant scrutiny since birth,” it reads. “Raised to be the closest of brothers, the last 18 months has seen a devastating breakdown of their once unbreakable bond.”

“With an unrivalled knowledge of court life and access to impeccable sources, Robert Lacey investigates the untold reality of the brothers’ relationship, explaining what happened when two sons were raised for vastly different futures and showing how the seeds of damage were sown as their parents’ marriage unravelled,” it continues.

The brothers now live physically further apart than ever, as Harry has moved to Los Angeles, California with Meghan since stepping down as a senior royal earlier this year. A source has previously said the brothers’ relationship was “forever changed” and they “won’t get back to the way they were.”

“Harry is looking straight ahead at his future with his family,” the insider added.