British Royal Navy decided in 1964 to adopt the US Navy F-4J Phantom for use on their aircraft carriers. However, large modifications were made on the F-4J, both in equipment and in structure. The largest modifications were perhaps to replace the General Electric J-79 engines used on the American Pahntoms by British Rolls Roys Spey 202/203 engines. This required changes in the fuselage (engine bay, inlet duct, bellmouth control systems, etc.) as the engines were not of the same size.