• A new book revealed more details about William and Harry's falling-out and the bullying allegations against Meghan Markle
  • Prince William was "horrified" after hearing the staffers' complaints against Markle, according to Robert Lacey
  • Prince Harry was "furious" when his brother believed staffers over him and his wife, the historian claimed

Prince William and Prince Harry's feud worsened after Meghan Markle was accused by a palace staffer of bullying, a new book has claimed.

In his new book "Battle of Brothers," British historian Robert Lacey covered Prince William and Prince Harry's falling-out and the bullying allegations against Markle that allegedly drove the Duke of Cambridge to initiate the separation of his household from Sussexes.

"Prince William went ballistic when he heard the ‘dossier of distress’ that [former communications secretary Jason] Knauf had gathered," Lacey wrote, according to an excerpt of the book published by U.K.'s The Times.

The royal biographer claimed that Prince William was "astonished and horrified" by the complaints against Markle when she was still a working member of the royal family and believed the former actress was "undermining some precious principles of the monarchy."

The future king was allegedly also concerned that Markle "seemed to be stealing his beloved brother away," according to the book. Middleton "had been wary of her from the start," Lacey wrote, citing an unnamed source.

After Jason Knauf, who worked for both the Cambridges and the Sussexes, submitted the email in October 2018 alleging that Markle had bullied staff, Prince William immediately called his younger brother to hear what he had to say, the book said. But Prince Harry was reportedly "furious" that his brother was more inclined to believe palace employees than him and his wife, according to Lacey.

The historian described the alleged "showdown" that followed as "fierce and bitter."

"He had seen it as his job to look out for Harry but this was the moment the protection had to stop," Lacey continued. "At the end of the day the British crown and all it stood for with its ancient traditions, styles and values — the mission of the monarchy — had to matter more to William than his brother did."

When the rift between the brothers worsened, Prince William reportedly initiated the division of their households, which saw Prince Harry and Markle moving from Kensington Palace to Frogmore Cottage.

"William wished to be separated from Meghan on a day-to-day basis — and that meant being separated from his brother as well," Lacey wrote. "'William,' says a friend, 'threw Harry out.'"

The royal historian noted that while many thought it was Prince Harry who orchestrated the separation, "the reverse was the case. It was William who made the decisive move."

After The Times' story on the bullying allegations came out in March, a representative for Markle issued a statement addressing what they called a "calculated smear campaign based on misleading and harmful misinformation."

"The Duchess is saddened by this latest attack on her character, particularly as someone who has been the target of bullying herself and is deeply committed to supporting those who have experienced pain and trauma. She is determined to continue her work building compassion around the world and will keep striving to set an example for doing what is right and doing what is good," the statement read.

Buckingham Palace also released a statement in March claiming it will investigate the claims against the duchess.

"Battle of Brothers: William, Harry and the Inside Story of a Family in Tumult" is set to be released Thursday.

Prince William, Kate Middleton, Meghan Markle and Prince Harry
Prince William, Kate Middleton, Meghan Markle and Prince Harry leave after attending Christmas Day Church service at Church of St Mary Magdalene on the Sandringham estate on Dec. 25, 2018 in King's Lynn, England. Getty Images/Stephen Pond