Princess Diana's brother Charles Spencer reportedly once warned his nephew Prince Harry against marrying Meghan Markle, according to a resurfaced book.

Royal author Tom Bower wrote in the 2022 book "Revenge: Meghan, Harry, and the War Between the Windsors" that before Prince Harry tied the knot with Markle in 2018, the 9th Earl Spencer voiced their family's "unease" about the former "Suits" actress.

In the book, Bower described the Spencer family's reaction after meeting the Duke of Sussex's then-fiancée, the Mirror reported.

After learning of the couple's engagement, Spencer -- who has been married three times and recently announced his divorce from his third wife -- "cautioned his nephew to reconsider his haste towards marriage," Bower wrote.

Charles Spencer, brother of Britain's Princess Diana
Charles Spencer, brother of Britain's Princess Diana arrives at Kensington Palace for the unveiling of a new statue to his sister on what have been Princess Diana's 60th birthday in London on July 1, 2021. Daniel Leal/AFP via Getty Images

But his uncle's advice to slow down his relationship with Markle reportedly "provoked a bitter reaction" from the Duke of Sussex, according to the book.

The author claimed Prince Harry was "disappointed" that the Spencer family did not see the similarities between Markle and his late mother and thought that the former actress would not be a good fit for the royal family.

"Harry assumed that Diana's family and friends would see a similarity between Diana and his fiancée," Bower wrote. "He was disappointed."

The author continued, "No one agreed that his vulnerable mother had anything in common with his girlfriend. More discomforting for him, they thought Meghan would not fit in with the royal family."

Enstarz could not independently verify the book's claims.

Prince Harry
Prince Harry, Duke of Sussex arrives for the Coronation of King Charles III and Queen Camilla at Westminster Abbey on May 6, 2023 in London, England. Andy Stenning - WPA Pool/Getty Images

Despite the family's alleged misgivings, Princess Diana's three siblings -- including Lady Jane Fellowes and Lady Sarah McCorquodale -- attended Prince Harry and Markle's wedding in St. George's Chapel at Windsor Castle on May 19, 2018.

Two years later, the Duke and Duchess of Sussex stepped back as working royals and moved to California.

But there appears to be no bad blood between Prince Harry and his mother's family. Earl Spencer and Lady Jane supported him last month as he led a service held at St Paul's Cathedral to mark the 10th anniversary of the Invictus Games.

A video shared by Hello! showed Prince Harry hugging his uncle and aunt after they arrived at the church.

Prince Harry and Meghan Markle
 Britain's Prince Harry, Duke of Sussex and his wife Meghan, Duchess of Sussex leave from the West Door of St George's Chapel, Windsor Castle, in Windsor on May 19, 2018 in Windsor, England. Ben STANSALL - WPA Pool/Getty Images
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Prince Harry, Meghan Markle, Princess Diana