• Charles Spencer and Prince Charles had a heated discussion over Princess Diana's funeral arrangements, a book claims
  • Spencer did not want Princes William and Harry to walk behind their mother's coffin during her 1997 funeral
  • Historian Robert Lacey explores Princes William and Harry's relationship in new book "Battle of Brothers"

Prince Charles said something "offensive" about his ex-wife, Princess Diana, during a call with her brother Charles Spencer, according to a new book.

In an excerpt published by People of royal author Robert Lacey's "Battle of Brothers: The Inside Story of a Family in Tumult," the son of Queen Elizabeth said the remark to Charles, 9th Earl Spencer, after Princess Diana died in a car crash in 1997.

According to the historian, Spencer had heated discussions with palace officials regarding Princess Diana's funeral arrangements. Spencer didn't want Prince William and Prince Harry to walk behind their mother's coffin during the procession to Westminster Abbey and believed that his late sister would not have approved of it.

"Prince Charles had no doubt that he should walk the long route with both his sons beside him. But Uncle Charles Spencer did not agree," Lacey wrote.

"He was already angry on his family’s behalf that his sister’s funeral had been hijacked into a royal occasion, and he was particularly opposed to the idea that his young nephews should have to walk the best part of a mile behind their mother’s coffin through the streets," he added.

Spencer aired this thought to Prince Charles, but the royal said something that infuriated Princess Diana's brother, per Lacey.

"Spencer felt quite sure that Diana would have been horrified at the idea of her sons having to endure such an ordeal. He had already told Charles as much," the author continued. 

The call "had ended with the earl slamming down the phone on his brother-in-law after Charles had made a particularly offensive comment about Diana," according to Lacey.

Neither Prince Charles nor Spencer has commented on the claims made in Lacey's book.

Despite Spencer's alleged disapproval, Prince William and Prince Harry joined their father, grandfather Prince Philip and their uncle for the procession on Sept. 6, 1997.

Lacey also touched on Prince Harry and Meghan Markle's relationship in his book and his falling out with older brother Prince William. He claimed that Prince Harry was furious when Prince William got Spencer involved while attempting to convince him to slow down in his romance with Markle.

According to the author, Prince Harry's response was a "brusque and offended pushback." Lacey added that the royal siblings clashed over something that mattered to them intensely, the future of the monarchy for Prince William and the love of his life for Prince Harry.

Meanwhile, Markle and Prince Harry could reunite with the royal family soon. An unnamed insider claimed to Us Weekly that the Sussexes are planning to spend the Christmas holidays with the royal family, though this plan is not yet final due to the COVID-19 pandemic.

Prince Charles and Princess Diana Prince Charles and Princess Diana are pictured attending a centenary service for the Royal College Of Music on Feb. 28, 1982 at Westminster Abbey, London. Photo: Fox Photos/Hulton Archive/Getty Images