• Prince William and Prince Harry reportedly quarelled over something that mattered to them intensely
  • The two princes allegedly had a confrontation in 2016 about Meghan Markle
  • Prince Harry and Markle are planning to spend the Christmas holidays with the royal family 

Prince Harry and Prince William's already fractured relationship allegedly worsened due to the younger royal's whirlwind romance with Meghan Markle, according to Robert Lacey in his new book, entitled "Battle of Brothers: William, Harry and the Inside Story of a Family in Tumult."

Princess Diana and Prince Charles were very close growing up, but many noticed that they became distant after Prince Harry dated and married Markle. Lacey attempted to explain their falling-out in his new book, with excerpts released this week via People.

"The fundamental conflict was between the two males who had known each other all their lives and had never hesitated to tell each other exactly what they thought and felt," Lacey wrote.

"William worried that his brother was moving too fast in his courtship — and he did not shrink from saying as much when Harry started talking about getting hitched to Meghan quite soon."

An insider also told the royal biographer that there was a confrontation between the two dukes in 2016. The Duke of Cambridge allegedly couched his questions concerning Markle.

However, Prince Harry was doubting his older brother's intention, whether he was truly concerned about his happiness or was thinking about the fortunes of "the Firm" given that Prince William is set to be the next king after their dad, Prince Charles.

"The response from Harry was a brusque and offended pushback," Lacey wrote.

When Prince Harry introduced Markle, many thought that the royal trio (Prince William, Kate Middleton and Prince Harry) would become the fab four. However, it did not happen because the two princes decided to split their households.

"Here we have a couple of brothers who loved each other dearly — most profoundly and genuinely, in fact—but who could give as good as they got when it came to a clash over something that mattered intensely to them,” he added.

"For William it was the future state of his monarchy — his sacred trust; while for Harry it was the love of the complex and captivating woman who had finally made sense of his life.”

Despite the rumored clash, Prince William and Middleton sent warm wishes to Prince Harry on his birthday last month on Instagram. One insider also claimed that Prince Harry and Markle are planning to spend the Christmas holidays with the royal family.

Prince William Meghan Markle and Prince Harry Prince William does not want Prince Harry and Meghan Markle to name their baby after their mom Princess Diana if they were having a girl. Pictured: Prince William, Markle and Prince Harry attend an Anzac Day service at Westminster Abbey on April 25, 2018 in London. Photo: Getty Images/Eddie Mulholland