- Princess Diana's biographer Andrew Morton told Insider that she overshadowed Charles.
- The king found her popularity "very difficult to take," according to the author.
Princess Diana's biographer Andrew Morton says King Charles struggled to accept that she overshadowed him when they were married.
Morton wrote the 1992 biography, "Diana: Her True Story," which shared details of the princess' unhappy marriage to Charles, his affair with Camilla, as well as Diana's struggles with her mental health.
Diana was secretly involved with the creation of the book through audio tapes which she sent to Morton for him to use as source material. After she died in 1997, Morton released a revised version of the book, "Diana: Her True Story — In Her Own Words," and for the first time admitted that the biography had been created with full cooperation from the princess.
Morton worked as a consultant on the fifth season of "The Crown," which depicted the book's creation.
Speaking to Insider, Morton described the different roles that Diana and Charles' current wife, Camilla, Queen Consort, had when it came to supporting him in his royal role. He said that while Camilla is the king's "cheerleader" and his "number two," Diana is somebody who overshadowed him.
"He has known Camilla for most of his lifetime. She has been a steady cheerleader waiting in the wings to console and comfort him in the way that he always really wanted somebody who would be a companion, but also who would be number two," Morton said.
As Insider previously reported, Charles and Camilla got married in 2005, but have known each other since the 1970s.
Charles' service in the Royal Navy, which began in 1972, put an end to their romance. However, they reconnected years later when they were both married to different people. Charles admitted to having an affair in 1994, saying in a TV documentary that he had remained faithful to Diana "until it became irretrievably broken down, us both having tried."
"Obviously Diana, never mind the personal difficulties they had, but on the public stage she overshadowed him. She was the star of the show and he wasn't. And it was something that he found very difficult to take," Morton continued. "Now he's the star of the show. He's king."
Diana was considered more popular than Charles during their marriage, and was even given the nickname "the people's princess."
This became evident during the couple's tour of Australia in 1983, where the public turned out in their thousands to welcome them — but it became apparent that most people only cared about seeing the princess.
"The crowds complained when Prince Charles went over to their side of the street during a walkabout ... In public, Charles accepted the revised status quo with good grace; in private he blamed Diana," Morton wrote in "Diana: Her True Story."
Buckingham Palace did not respond to Insider's request for comment.