The late princess, who would have turned 60 on Thursday, didn’t divulge why she thought her youngest son would be a better fit for the top job than second-in-line to the throne Prince William.
But it has been claimed that William often confided in her that “he didn’t want to be king”.
A young Harry is said to have cheekily replied on one occasion: “If you don’t want the job I’ll have it.”
According to royal expert Robert Jobson, this led to Diana privately calling Harry GKH – or Good King Harry.
He told the Channel 5 programme William & Harry: Princes At War?: “She used to refer to Harry as GKH (Good King Harry) because she thought he’d probably be better equipped for the role in the future than William.”
Meanwhile, TV broadcaster Jeremy Paxman also said Diana talked about her sons’ futures during a lunch meeting.
He explained: “We talked about our children and she said William often told her that he didn’t really want to be king, and then Harry would say, ‘If you don’t want the job I’ll have it’.”
In 1995, during her famous Panorama interview with Martin Bashir, the princess was asked if the title of king should go directly to William, who was then aged 13.
She said: ‘Well, then you have to see that William’s very young at the moment, so do you want a burden like that to be put on his shoulders at such an age? So I can’t answer that question.”
However, in the bombshell chat, she did question Prince Charles ‘ suitability to being king.
When asked if she believed her ex-husband would ever take the title she replied: “There was always conflict on that subject with him when we discussed it, and I understood that conflict, because it’s a very demanding role, being Prince of Wales, but it’s an equally more demanding role being king.
“And being Prince of Wales produces more freedom now, and being king would be a little bit more suffocating.
“And because I know the character I would think that the top job, as I call it, would bring enormous limitations to him, and I don’t know whether he could adapt to that.”