LONDON – Prince Charles was a “hopeless” lover who rarely slept with Princess Diana, according to a never-before-seen video to be shown next week.

The late princess also claims her husband brazenly admitted carrying on an affair with his mistress and Queen Elizabeth brushed off her complaints.

Diana recalls – in the 1993 tape made by her former voice coach – how “absolutely traumatized” she was when a TV interviewer asked the newly engaged couple if they were in love and Charles replied, “Whatever ‘in love’ means.”

After their marriage, their sexual relationship was “odd,” with Charles sleeping with her about once every three weeks.

“It followed a pattern,” she says – suggesting Charles used to see his lover, Camilla Parker Bowles, once every three weeks before he married Diana.

After about four years, sex between the royal couple “fizzled out” altogether, she says.

The 83-minute video will be shown in two hour-long specials on NBC’s “Dateline” – on Monday and a week later on Dec. 6.

On the tape, Diana says she confronted Charles over Parker Bowles but he said it was his birthright to fool around.

“I refuse to be the only Prince of Wales who never had a mistress,” she recalls him saying.

In tears, she went to the “top lady” – Queen Elizabeth – to complain about her husband, but was rebuffed. “I don’t know what you should do,” the queen replied, Diana says. “Charles is hopeless.”

The most sinister revelation by Diana, who was killed in a 1997 car crash at age 36, concerns her bodyguard, Barry Mannakee, who she says was murdered in 1987 for having an affair with her.

She calls Mannakee, a married police officer, “the greatest fella I’ve ever had.”

Mannakee was moved out of the job when rumors of a liaison with Diana surfaced. Less than a year later, he died in a mysterious motorcycle accident.

“It was all found out and he was chucked out. And then he was killed. And I think he was bumped off. … We’ll never know,” says Diana. She calls his death “the biggest blow of my life.”

The tapes come from a collection of 20 made by Peter Settelen, 53, who was the princess’ voice coach in 1992.

Although Settelen had vowed never to sell them, his lawyer, Marcus Rutherford, said Settelen changed his mind partly because of his mounting legal bills.

But Rutherford, who wouldn’t reveal what NBC paid for the tapes, said it’s better for people to hear Diana’s own words than tell-all accounts by others.

“She’s telling her own story,” Rutherford said.