Earl Spencer’s book, Swan Song: Diana, My Sister, is due to be published next week and is expected to be highly critical of the Royal Family.
In further extracts published on Thursday, he claimed how the King “sounded giddily elated” when he called him after Diana’s death in a car crash in Paris on 31 August 1997, alleging Charles sounded like a “lottery winner”.
Earl Spencer recounts being alerted to the crash by a phone call from his sister Jane in the middle of the night at his home in Cape Town, South Africa.
A call from Charles was among those that followed, and he alleges: “Unlike my other callers, who were stunned, barely able to articulate their condolences and offers of help, he sounded giddily elated – like a lottery winner, I remember thinking at the time.”
The death of Diana, a hugely popular figure, created a volatile atmosphere in the country at the time, with the royals accused of failing to show feeling and empathy, and seeming to be out of touch with the public mood.
Earl Spencer’s emotional and defiant speech at her funeral added to that cloud of grievance hanging over Diana’s death at the age of 36.
Prince Harry has previously said that walking behind his mother’s coffin aged 12 was something no child “should be asked to do”.
In 2017, he told the he didn’t “have an opinion whether that was right or wrong”, but “looking back on it”, he was glad to have been part of the day.
The full interview with Laura Kuenssberg will be on iPlayer on Sunday from 16:00 BST and on One at 18:30 BST.