Produced as a turbine-powered development of the American Sikorsky S-58, the initial British production version was the Wessex HAS Mk 1 for the Royal Navy, entering service in 1961. First flown in January 1962, the RAF variant, the HC Mk 2, was a high-performance development of the Mk 1, with two coupled Bristol Siddeley Gnome turboshaft engines. It was intended for transport, ambulance and general purpose duties, including carrying 16 fully-equipped troops or a 4000lb underslung load (such as a 105mm Pak Howitzer) and ground assault with Nord SS-11 anti-tank missiles and machine guns. It entered service, with No 18 Squadron at RAF Odiham, in January 1964, with 71 delivered. The HC Mk 2 became a familiar sight on anti-terrorist operations in Northern Ireland, and supported UN Peacekeeping forces in Cyprus.
The HAR Mk 2 variant was used by detached Flights of No 22 Squadron RAF for SAR duties around the UK coast from 1976, twelve aircraft, converted from
HC Mk 2s with the addition of a winch above the starboard door and a bright yellow high-visibility colour scheme, replacing Westland Whirlwinds.
The last RAF Wessex helicopters (Cyprus based HC Mk 2s) retired in 2003.