But Charles could not pull out of the marriage and the Prince is said to have hoped their wedding would solve the issues.
Writing in The Duchess: The Untold Story, Ms Junor said: “Charles was not convinced he was doing the right thing in marrying Diana but there was no way out and, bolstered by the hope that things would be different once they were married, he put a brave face on it.”
Ms Junor said Diana’s personality changed after she became engaged to Charles, describing her “bizarre behaviour”.
The royal author said: “During those months before the wedding, the bouncy young girl so many people had encouraged Charles to marry changed radically.
“She lost weight. The blue dress she wore for the engagement was a size fourteen. Her waist was twenty-nine inches.
“By the time she walked up the aisle, her waist had shrunk to twenty-three and a half inches.
“But as worrying as the weight loss was her mental state – and the Prince wasn’t the only one to notice her bizarre behaviour.
“The girl who had always seemed to be so easy, funny and cheerful, suddenly became moody, wilful and unpredictable.
“There were terrifying rages, temper tantrums and hysterical tears for no apparent reason, and her moods changed in a flash.”
Diana and Charles announced their engagement on February 24 1981.
Charles previously dated long-term love Camilla in the early 1970s, who married Andrew Parker Bowles in 1973.
But ahead of his engagement to Diana, the then 32-year-old Charles was under increasing pressure to get married and fulfil his duty as heir to the throne.
Despite his concerns, Charles married Diana on July 29 1981 in a spectacular ceremony at St Paul’s Cathedral, watched by an estimated 750 million viewers across the world.
However the couple separated in 1992 and divorced in 1996 after the Queen wrote to the pair urging them to end their marriage following years of tensions.
Diana died a year later in a car crash in Paris. Charles went on to marry Camilla in 2005.