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The 125 started life in the early 1960s as a De Havilland project before coming within the Hawker Siddeley group. The final Viper turbojet powered 125s built were the 600A and 600B series. They were powered by RollsRoyce Viper 601-22 turbojets and featured a stretched fuselage that typically seated eight passengers, or up to 14 in a high density configuration. The 125-600s had a longer vertical tail and ventral fin than the 125-400, and housed a fuel tank in the extended dorsal fin for increased fuel capacity.

The first 600 flew on 21 January, 1971 and it became the standard production model until the Garrett TFE-731 turbofan powered 700 series was introduced in 1976. Some Series 600s were re-engined with TFE-731s as HS-125-F600s.