The alleged incident happened while the monarch was on a walkabout during a visit to Luton.
Image source, PA MediaA man has been arrested on suspicion of common assault after an egg was reportedly thrown in the direction of the King.
The alleged incident happened while King Charles was on a walkabout in Luton town centre on Tuesday.
Bedfordshire Police said a man in his 20s was detained and is in custody.
The monarch was steered away from crowds by his security staff before continuing to meet members of the public.
The latest incident comes just weeks after a 23-year-old student was arrested after eggs were thrown at the King and Queen Consort during their visit to York.
The royal couple had just arrived in the city on 9 November to unveil a statue in honour of the late Queen at York Minster when four eggs were thrown, all of which missed.
Many of those who greeted King Charles at Luton Town Hall wished him a “Merry Christmas”, while some had brought presents to give to him.
The King also met community leaders, including from the Ghana Society, the Royal British Legion and the Luton Town football academy, as well as officially opening the Guru Nanak Gurdwara temple.
Later, the King boarded the DART (Direct Air-Rail Transit) – a new airport shuttle which will connect people travelling from Luton Airport Parkway station to the terminal.