Never before seen letters have revealed what Prince Philip really thought of Princess Diana's divorce from Charles - and it turns out he never "dreamed" Charles would leave her for Camilla.

When Prince Charles and Diana split 25 years ago, the world was shocked.

It is claimed the catalyst was the Princess of Wales' Panorama interview with Martin Bashir, where she said there were "three people" in her marriage, according to The Sun.

Following the scandalous revelation the Queen is said to have written to the royal couple telling them to divorce.

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The Prince and Princess of Wales on the balcony of Buckingham Palace on their wedding day, 29th July 1981. Photo / Getty Images
The Prince and Princess of Wales on the balcony of Buckingham Palace on their wedding day, 29th July 1981. Photo / Getty Images

However, recently unearthed letters from 1992 reveal the Duke of Edinburgh took Diana's side when things "got messy" between her and Prince Charles.

Philip also wrote about his disapproval of their son's love life, a stance also shared by the Queen.

"Charles was silly to risk everything with Camilla for a man in his position," Prince Philip wrote to Diana.

"We never dreamed he might feel like leaving you for her. I cannot imagine anyone in their right mind leaving you for Camilla. Such a prospect never even entered our heads."

"With fondest love, Pa."

Snippets of Diana's letters to Prince Philip suggest a close bond between the pair. Photo / Handout
Snippets of Diana's letters to Prince Philip suggest a close bond between the pair. Photo / Handout
The late princess is open to advice from her former father-in-law. Photo / Handout
The late princess is open to advice from her former father-in-law. Photo / Handout

In the letter, he also expressed sympathy and support to his daughter-in-law and offered to mediate the "war" between her and her estranged husband.

"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 the letter.

"But I am quite ready to concede that I have no talent as a marriage counsellor."

Although he took Diana's side throughout the letter, Prince Philip also made it clear he did not support her having a new romantic relationship, writing: "We do not approve of either of you having lovers."

Diana said she feels her and Prince Philip are beginning to understand each other. Photo / Handout
Diana said she feels her and Prince Philip are beginning to understand each other. Photo / Handout

He also tried to put some blame on her for the marriage breaking down writing: "Charles's relationship with Camilla had nothing to do with your behaviour towards him in your marriage?"

According to Princess Diana's private secretary at the time, Patrick Jephson, her support from the in-laws meant a lot to her.

"Here at last was written proof that this was acknowledged, recognised, and there was sympathy for her," he said.

Details or the royal couple's rocky marriage were revealed to the public for the first time in the same year these letters were written.

Fabulous Digital has contacted Buckingham Palace for comment.