No other details were immediately available.
Charles, 73, met dozens of people during a Wednesday evening reception at the British Museum.
The heir-to-the-throne previously contracted the coronavirus in March 2020. Officials said he had mild COVID-19 symptoms then.
Charles and his wife, Camilla, isolated at Queen Elizabeth II’s Balmoral estate in Scotland at the time.
Spain’s King Felipe VI, 54, and Denmark’s Queen Margrethe, 81, also tested positive for the coronavirus this week.