Prince Philip tried to help Princess Diana mend her marriage with Prince Charles.

The late Princess of Wales had the sympathy of her father-in-law, Prince Philip. In fact, the Duke of Edinburgh had been sending her letters amid her rocky marriage with Prince Charles.

In one letter to Princess Diana, Prince Philip called his own son "silly." Queen Elizabeth II's husband was not happy about the Prince of Wales' decision to forego everything for his affair with Camilla Parker Bowles.

"I can only repeat what I've said before if invited, I will always do my utmost to help you and Charles to the best of my ability," he wrote in a letter to Princess Diana on July 7, 1992 (via Express). "But I am quite ready to concede that I have no talent as a marriage counselor."

"We do not approve of either of you having lovers," Prince Philip continued. "Charles was silly to risk everything with Camilla for a man in his position. We never dreamed he might feel like leaving you for her."

According to Sarah Gristwood, Prince Philip had openly supported Prince Charles' ex. Just like Princess Diana, the duke is regarded as an innovator.

"Never mind how Camilla might feel 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," Gristwood said.

Meanwhile, Princess Diana said in a past interview that Prince Philip advised Prince Charles to get back with Camilla if their marriage doesn't work.

"My father-in-law said to my husband, 'If your marriage doesn't work out, you can always go back to her after five years,'" Princess Diana said in a 2017 Channel 4 special that utilized taped conversations between the Princess of Wales and her voice coach. "Which is exactly—I mean, for real I knew that it had happened after five—I knew something was happening before that but the fifth year I had confirmation."

In related news, Princess Diana topped the poll of most iconic women. The People's Princess beats her daughters-in-law Kate Middleton and Meghan Markle.

Princess Diana received fifty-seven percent of the vote. The Duchess of Cambridge comes second with 33 percent. On the other hand, the Duchess of Sussex lags behind on the sixth spot.

Prince Philip and Prince Charles
Prince Philip reportedly called Prince Charles "silly" in his letter to Princess Diana. Pictured: Queen Elizabeth II, Prince Philip and Prince Charlesf rom the Balcony of Buckingham Palace during the 'Trooping of the Colour' June 16, 2001 in London. Getty Images/Anthony Harvey