In his book A Royal Duty, Paul Burrell, the late Princess Diana's close friend and former butler, revealed the promise that the young prince made to his mother while he was just 14 and studying at Eton College following his parents' divorce and the stripping of his mother's royal title.
"She told me how he had sat with her one night when she was upset over the loss of HRH, put his arms around her and said, 'Don't worry, Mummy. I will give it back to you one day when I am king,'" Burrell wrote in his book, Mirror reports.
The late Princess' divorce from Prince Charles was a contentious one, with the couple formally separating in 1992, though their divorce was not finalized until almost four years later in 1996, a delay that was credited to disagreements over money, access, and most prominently Diana's title of "Her Royal Highness."
As the mother of the future king, Diana wished to keep her title, the retaining of the Princess' HRH styling something that Queen Elizabeth II was reportedly fine with, though Prince Charles was "adamant" that she be stripped of it, according to a report from The New York Times. The stripping of the title meant that while Diana would still be a member of British nobility, she would be separated from the royal family, requiring her to curtsy to ex-husband and children.
However, Prince William never had the opportunity to keep his promise, as Princess Diana passed away only a year later on August 31, 1997 in a car accident in Paris. Two decades later, Prince William opened up about that night and the regret he held regarding his last phone call with his mother.
"At the time Harry and I were running around minding our own business, you know, playing with our cousins and having a very good time. I think Harry and I were just in a desperate rush to say goodbye, you know, see you later and we're going to go off," William revealed. "If I'd known now obviously what was going to happen, I wouldn't have been so blasé about it and everything else. But that phone call sticks in my mind quite, quite heavily."
Prince William, along with the rest of the royal family, has continued to honor Princess Diana's legacy by continuing her charitable works, including supporting the homeless, normalizing mental health conversations, and fighting the stigma around HIV/AIDS.