Prince Philip reportedly preferred Princess Diana for Prince Charles over Camilla Parker Bowles.

The Duke of Edinburgh had always wanted Princess Diana's marriage to the Prince of Wales to work. In fact, he wrote a letter to the late Princess of Wales in which he called his own son, Prince Charles, "silly" for leaving the mother of his two children to be with Camilla.

"One of the original royal outsiders quietly entered the conversation," Netflix's documentary "The Royal House of Windsor" revealed. "Prince Philip, a man who had given up his naval career to stand in his wife’s shadow, wrote Diana a string of deeply personal letters."

"Never mind how Camilla might felt about that he was being supportive towards Diana and no one knew better than Prince Philip just how hard it was for an outsider to come into the Royal Family," historian Sarah Gristwood said about Prince Philip's letter to Princess Diana (as quoted by Express).

Prince Philip and Prince Charles have a lot of differences. In fact, the 97-year-old royal is reportedly not confident with the heir taking over the throne.

According to Adam Helliker, Prince Philip is "scathing" and not supportive of Prince Charles' "harmful" plans to modernize the monarchy. "Philip has fulminated against Charles's vision of a slimmed-down Royal Family which he wants to eventually be independent of taxpayers' support, and has called his son 'wilful and petulant,'" Helliker wrote.

"He has been particularly scathing over Charles's plan - which he is said to intend to implement early in his reign - to turn Buckingham Palace into a space for the nation, while he relocates the sovereign's base to Windsor Castle," he added.

Prince Philip and Prince Charles have reportedly had a strained relationship. At one point, the future king disappointed his own father when he snubbed a state dinner hosted by a Chinese government in 1999.

"This move turned to be a source of considerable frustration and dismay to Prince Philip, who's led an intensely disciplined life, with which he's kept his mouth largely shut, under intolerable stresses and strains, and he's been this absolutely rock-like support," Sir Max Hastings revealed.

Prince Philip, Prince Charles and Queen Elizabeth II
Pictured: Prince Charles, Prince Philip and Queen Elizabeth II from the Balcony of Buckingham Palace during the 'Trooping of the Colour' June 16, 2001 in London. Getty Images/Anthony Harvey