As some others have pointed out, there are some cases where illegitimate children of royal men in England have received surnames that start with
Fitz- such as
Fitzroy or Fitzclarence.
However, there have been a lot of commentators who have therefore jumped to the conclusion that ALL families with surnames starting with
Fitz- are descended from royal bastards, and that is definitely NOT the case. The huge majority of persons with
Fitz- surnames have no royal connection.