Work on the project for the tank meant to replace the Centurion began in the early 1950s, but the first prototypes only appeared in 1959. The FV4201 Chieftain entered service in May 1963 and was produced until 1985. The Chieftain Mk. 6 is the Chieftain Mk. 2 with a new power unit and an altered bow gun. A total of 2,265 vehicles were produced in various modifications.