The breed wasn’t officially recognised in the U.K. until 1945. Prior to that it was considered to be the same as an English Toy Spaniel, known as the King Charles Spaniel by The Kennel Club.
An American Toy Spaniel enthusiast named Roswell Eldridge was responsible for the emergence of the Cavalier King Charles Spaniel. He offered a substantial amount of money to anyone who could produce an old-fashioned type as depicted in 16th, 17th and 18th century paintings. That duly happened and the Cavalier King Charles Spaniel Club was established in 1928.