The King laughed off an awkward comment about Prince Harry as he carried out engagements in East London on Wednesday.
The monarch was greeting students at the University of East London (UEL) at its Stratford campus, when one asked: "Can you bring back Harry?"
Charles replied: "Who?" after initially mishearing, before bursting into laughter when he understood what the man had said.
Charles toured pioneering hospital training wards at UEL
The King's youngest son, Harry, lives in Montecito with wife, Meghan, after the couple stepped back from royal duties in 2020.
Last month, Harry released his tell-all memoir, Spare, in which he detailed his strained relationship with members of the royal family, including his father, stepmother, Camilla, and older brother, Prince William.
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Buckingham Palace did not respond to any claims or allegations made in the book or the Sussexes' six-part Netflix docuseries, which aired in December.
It is not known if Harry and Meghan will attend the King's coronation on 6 May, but the Duke addressed the matter during his ITV interview with Tom Bradby earlier this year.
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During his visit to the university, Charles toured pioneering hospital training wards, encountering mannequins that "speak", "breathe" and "blink".
The King had earlier met three infants and their mothers at the hub’s baby development lab which researches how living in urban areas affects children’s early years.
King Charles meets Ukraine's President Zelenskyy at Buckingham Palace
Earlier in the day, the King and Queen Consort met London’s Bangladeshi community during a trip to Brick Lane.
The monarch also met with Ukraine's President Zelenskyy for the first time at Buckingham Palace on Wednesday.
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