The British monarch made her remarks after a desperate Princess Diana turned to her for some marriage advice.
Prince Charles had told his young wife that he 'refused to be the only Prince of Wales who never had a mistress' after she confronted him about his affair with Camilla.
The revelations come from interviews with her voice coach Peter Settelen.
And Diana was given little hope when the Queen told her quite simply: Prince Charles is hopeless.
The Princess, who tragically died in a 1997 car crash in Paris, made the revelations in the documentary, Diana: In Her Own Words.
During the sessions with Settelen, Diana reveals that she asked Charles about why Camilla Parker Bowles – now his wife the Duchess of Cornwall – was in his life.
She said: "I remember saying to my husband, ''Why? Why have you got this lady around?'' And he said, ''Well, I refuse to be the Prince of Wales who never had a mistress.''
On her showdown with the Queen she revealed: "So I went to the top lady, sobbing and I said, ''What do I do? I'm coming to you, what do I do?''
"And she said, ''I don't know what you should do, Charles is hopeless''. And that was it, that was help.'
Surprisingly Diana also discusses how she and Charles only spent time together on thirteen occasions before they got married.
And she was shocked when he suddenly got romantic with her when they were discussing the death of Earl Mountbatten.
She said: "We were at a barbecue that night talking about Mountbatten and his girlfriend and I said, 'You must be so lonely.'
"I said, 'It's pathetic watching you walking up the aisle with Lord Mountbatten's coffin in front, ghastly. You need someone beside you...'
"Agggh. Wrong word! Whereupon he leapt upon me and started kissing me and everything and waves arms urrgh...You know, this is not what people do."
However, this impulsive passion from him subsided shortly after they got married and he kept seeing Camilla.
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