Robert Lacey, author of Battle of Brothers and consultant to Netflix's The Crown, discusses the brothers’ ongoing conflict

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Royal historian Robert Lacey has updated his book Battle of Brothers: William, Harry and the Inside Story of a Family in Tumult, and he admits he was taken aback by the stories he uncovered about the rift between Prince William and Prince Harry.

"In updating this story, I am saddened about the depth of the rift and the animosity between the two of them," Lacey tells PEOPLE in this week's issue.

Lacey wrote the first edition of the book last year but has spent the time since updating it and adding more chapters.

At the same time, says Lacey, "I have also come to feel there are solid, long-term reasons for this being resolved in the future."

royal wedding 2011
Prince Harry and Prince William at the 2011 royal wedding
| Credit: Andrew Milligan/Getty

One of those moments for a step towards reconciliation may come next week when the brothers are united at the unveiling of a statue dedicated to their late mother Princess Diana, who would have been 60 on July 1.

Prince Harry and Prince William sussex cambridge
Prince Harry and Prince William
| Credit: Tolga Akmen - WPA Pool / Getty Images

According to some of Lacey's new chapters, things worsened again in October 2018 when William, 39, learned of allegations from staffers who said they had been bullied. (When the bullying claims first surfaced in The Times in March, the Duchess of Sussex's office strongly denied the allegations and said they were part of a smear campaign against the Duchess, 39. The palace set up an independent inquiry, which is still "ongoing," a source confirms.)

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The tensions have also devastated members of the royal family - including their father, Prince Charles.

Prince Charles, Prince of Wales, Prince William, Duke of Cambridge and Prince Harry
Credit: Samir Hussein/WireImage

The future king, 72, has historically had complicated relationships with his sons and is finding it hard to intervene as increasingly painful revelations about the roots of the brothers' discord continue to come to light.

Charles is "shellshocked by it all," a friend tells PEOPLE exclusively in this week's issue. "He is very hurt and upset [by the ongoing fallout]."

The book is out in the U.K. on Thursday, June 24, and is set to be published by HarperCollins out in the U.S. on October 19.