Prince Philip stood by Diana while she suffered in her stormy marriage to Charles

Princess Diana may have had strained ties with many members of the British royal family, but her relationship with Prince Philip remained special.

According to a royal expert, the late Princess of Wales was extended support by her father-in-law during the unstable years of her married life with Prince Charles.

Author of Kensington Palace: An Intimate Memoir from Queen Mary to Meghan Markle, Tom Quinn spoke to Fox News following the demise of the Duke of Edinburgh on Friday and looked back at his relationship with his late daughter-in-law.

“Philip spent seven decades walking behind the queen both literally and metaphorically – but he was the backbone of the royals’ private life, supporting Diana during her most difficult periods,” said Quinn.

“He also admonished his sons, including Prince Charles, when he felt their behaviour was not good enough,” he continued.

Quinn claimed that the Duke had stood behind Diana when it was discovered that the Prince of Wales was cheating on her with his then ex-girlfriend Camilla Parker Bowles.

Philip had reportedly penned a number of letters to Diana in 1997, giving her advice and providing her with support.

According to Woman and Home magazine, Philip had reportedly written in one of the letters: “If invited, I will always do my utmost to help you and Charles to the best of my ability. But I am quite ready to concede that I have no talent as a marriage counsellor!”

Quinn mentioned that in another letter, Philip had expressed his bafflement at his son’s decision to abandon Diana to be with Camilla.

“He told her he thought Charles was mad to have even thought of abandoning her,” he said.