The King was commended for pouring the "perfect" pint of Guinness on a visit to mark the opening of a new brewery in London.
Charles tried his hand at pulling pints on a tour of the Guinness Open Gate Brewery London in Covent Garden on Thursday.
Guided by Guinness experience ambassador Leo Ravina, the King was advised to hold the glass emblazoned with the iconic harp at a "45 degree angle - very important - it's not 46 or 44, precisely 45".
In keeping with tradition, Charles stopped the pour when the glass was three quarters full to allow the Irish stout to settle.
The King chatted to his guide before completing the pint.
At the end of the process, Mr Ravina said "that's a perfect pint of Guinness".
"If you say so," the King said.
The finished pint - which took about 120 seconds to pour - was decorated with the words "Merry Christmas" and a sprig of holly by a machine using edible dyes.
After marvelling at the decorations, Charles's first sip left his upper lip frosted with a Guinness moustache.
The King also sampled some of the beers produced by a brewery in the building's basement, having a sip of a porter, the festively named "winter warmer" and an apricot sour, which Charles remarked did not taste like a beer.
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Mr Ravina said afterwards: "He did great, that was an ideal pint of Guinness. He's got the job - he's hired."
Charles provided an update on his cancer diagnosis last week, revealing that his treatment can be reduced in the new year because of early diagnosis.
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The monarch, who announced he had been diagnosed with an unspecified type of cancer in February 2024, also urged Britons to take up available screenings for the disease.
The Royal Family has endured a turbulent year after the King stripped his brother, Andrew Mountbatten-Windsor, of his remaining titles in October over controversy concerning his links to paedophile financier Jeffrey Epstein.
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