Prince Philip was "determined" to save the marriage between his son, Prince Charles, and the late Princess Diana, with the duke writing "tough but tender" letters in a bid to help their strained relationship, according to his biographer.
Prince Philip, the Duke of Edinburgh, died on Friday. He was 99.
Looking back on the duke's life in an article for the Daily Mail, royal biographer Gyles Brandreth said Philip and his wife, Queen Elizabeth II, would often urge Charles and Diana to compromise during rocky patches. The couple married in 1981 when Diana was 20 and Charles was 32. The couple had two children, Prince Harry and Prince William, before divorcing in 1996 after a tense and bitter union.
The pair both had affairs during their marriage. Diana admitted to one of hers during her 1995 interview with BBC's Panorama, saying she "was in love" with cavalry officer James Hewitt.
Philip also wrote a series of letters that included "tough love" to his son and daughter-in-law, Brandreth said.
"His underlying message was always positive. As for his purpose, it was characteristically practical: to guide her towards the possibility of a reconciliation with Charles," the biographer wrote.
"In short, Philip confronted his daughter-in-law with home truths, and invited her to think about her marriage, long and hard."
In one letter, Brandreth says Philip penned the things Charles and Diana had in common, such as activities, interests, and people.
In another, he said to Diana: "I cannot imagine anyone in their right mind leaving you for Camilla."
"I doubt that the trained marriage counsellors could have done very much more," Brandreth said.
Diana's former butler, Paul Burrell, also acknowledged in the Daily Mail story the duke "did more to save the marriage than Prince Charles".
Philip's funeral will be held on Saturday (UK time) at St George's Chapel on the grounds of Windsor Castle.
Due to the ongoing pandemic, funerals in Britain are required to have no more than 30 attendees, regardless of whether it is held indoors or outdoors. Guests must stay at least two metres apart from those not in their household.
Because of this rule, the Queen may be forced to sit by herself as the family is not all in the same bubble.