Sir Rod Stewart has told the King he was "absolutely superb" during his recent state visit to the US, adding that His Majesty "put that little ratbag in his place".
The singer raised the four-day trip with Charles and Camilla during an event on Monday celebrating the 50th anniversary of the King's Trust at the Royal Albert Hall.
Sir Rod said: "May I say, well done in the Americas. You were superb, absolutely superb; put that little ratbag in his place."
Charles appeared to laugh off the comment, as Rolling Stones guitarist Ronnie Wood smiled next to them.
The King addressed the US Congress during the visit, in which he and Camilla were hosted by Donald Trump and his wife Melania.
Charles touched on topics such as NATO, Ukraine and the environment – all topics which appear to be out of favour with the White House.
Later, in a speech at a state dinner, the King joked to his hosts: "If it wasn't for us, you'd be speaking French!"
At Monday's event, Sir Rod appeared to speculate about how Charles's speech had been received, telling His Majesty: "Exactly, that's it – it went right over his head, right over his head."
The singer also approached Camilla to enforce his point about the US trip.
He told her: "I was just congratulating your husband on his wonderful performance in the Americas – so great, so brave, so proud."
As they arrived on the red carpet, Charles and Camilla were greeted by TV presenters Anthony McPartlin and Declan Donnelly, who were hosting the evening.
Camilla was wearing a purple lace cocktail dress by Fiona Clare, with a crepe magenta cape by Dior, and amethyst and diamond earrings which once belonged to the Duchess of Kent, Queen Victoria's mother.
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Some of the young people helped by the trust were presented with awards by celebrities, including singer Alesha Dixon and Emily In Paris actress Lily Collins.
The King and Queen also spoke to performers Rita Ora, Anne-Marie, Craig David, Skye Newman and Jools Holland.
They were joined by British actors Benedict Cumberbatch and Sir Idris Elba, and Hollywood star George Clooney and his lawyer wife Amal.
The trust says it has helped more than 1.3 million young people across the UK in the past 50 years as part of its commitment to ending youth unemployment and helping more young people create better futures for themselves.
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