A man in his 20s has been arrested on suspicion of common assault after an egg is believed to have been thrown in the direction of Britain's King Charles during a walkabout on Tuesday, PA Media reported, citing local police.
A police spokesperson confirmed a man had been arrested.
Charles, who came to the throne in September after the death of his mother Queen Elizabeth, was visiting Luton, to the north west of London.
Buckingham Palace declined to comment.
Last month, a man was detained by police after eggs were thrown at Charles and Camilla, his wife and queen consort, as they carried out an engagement in northern England.
Royals have previously faced such protests. Eggs were also thrown at Elizabeth's royal car in 2002 when she visited Nottingham, central England, and anti-British protesters threw eggs at Charles during a walkabout in central Dublin in 1995.
