Earl Spencer’s interview with the was his first since the Palace issued a strongly-worded response to his claims about King Charles.
That statement said the King was “mindful that the pain of fraternal grief can cloud reason, affect judgement and colour memory in ways others do not recognise, even many years after such a loss”.
Earl Spencer, in comments first reported on Friday, reacted to it by saying: “Well, is he furious? Is he furious or is he embarrassed?”
He added: “I’ve only ever had two calls on the line with Prince Charles at any stage of my life.
“I was hyper alert to what he was saying and those are precisely the words that he said and I promise that to you now. Genuinely, that is what he said. I don’t hear him saying he didn’t say that.”
Pressed on whether he had evidence to back up the claim, he said he told family and friends at the time what had been said “because it was so monstrous”.
In his response to the Palace disputing his account, Earl Spencer, who shares his first name with the King, said: “They’ve sort of smudged that one, haven’t they? They’ve said that recollections may vary.”
When asked which Charles the public was meant to believe, he said: “I’m telling the truth.”
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, alleging Charles sounded like a “lottery winner”.