The tapes were recorded by the princess at Kensington Palace between 1992 and 1993, as she opened up to her tutor Peter Settleten.
They give an insight into the royal’s state of mind during her marriage.
Diana had been married to Prince Charles for 12 years by the time the tapes were recorded.
She said: “Well there was [ѕех.] There was. There was. But it was odd, very odd.
“But it was there then it fizzled out about seven years ago, six years ago, well seven, because Harry was eight.+
“Instinct told me, it was just so odd. I don’t know there was no requirement for it from his case.
“Sort of once every three weeks and I kept thinking.
“And then I followed a pattern, he used to see his lady once every three weeks before we got married.”
The Princess of Wales had both of her children, princes William and Harry by the time she’d recorded these tapes.
She was also painfully aware of Charles’ ongoing affair with flame Camilla Parker Bowles.
Despite a public outcry, Channel 4 is set to broadcast the secret tapes for the first time on UK television, on August 6 at 8pm.
They expose her childhood, the advice the Queen gave her about Charles and how she felt about Mrs Parker Bowles.
In a vivid portrait of her troubled union, she details how years before she wed Charles in a lavish ceremony in 1981 he “was all over her”.
She said: ”I said, you must be so lonely. I said it’s pathetic watching you walk up the aisle at St Paul’s with Lord Mountbatten’s coffin in front.
“I said, you know, ‘Ghastly, you need someone beside you’. Oh. Wrong word.
“Whereupon he leapt upon [me], he started kissing me and everything and I thought, ‘Ahh, you know, this is not what people do,’ and he was all over me for the rest of the evening, followed me around, everything.
“A puppy. And, yeah, I was flattered, but it was very puzzling.
“He wasn’t consistent with his courting abilities. He’d ring me up every day for a week and then he wouldn’t speak to me for three weeks, very odd.
“And I’d accepted that and I thought fine, he knows where I am if he wants me.
“And then the thrill when he used to ring up was so immense and intense, drive the other three girls in my flat crazy. But no, it was all, it was odd.”
The young, beautiful, royal first met Charles, when she was 16 on November 1977. At the time, he was dating her older sister Lady Sarah Spencer.
Diana, who loved reading romance novels as a teen, revealed that her love affair with her prince was a huge disappointment from the start.
She said on the tapes: “I was brought up in the sense that you know, when you got engaged to someone, you loved them.
“The most extraordinary thing is we had this ghastly interview the day we announced our engagement and this ridiculous ITN man said, ‘Are you in love?’
“I thought what a thick question so I said, ‘Yes, of course we are,’ in the sort of Sloane Ranger I was, and Charles turned round and said, ‘Whatever in love means’.
“That threw me completely. I thought what a strange question and answer. God. Absolutely traumatised me.
“No, I didn’t dare (ask him about it). Must have been, yeah (frightened). We met 13 times before we got married (laughs).”