From 1953 onwards, the first three prototype aircraft were hand-built at Samlesbury. These aircraft had been assigned the aircraft serials "WG760", "WG763", and "WG765" (the static airframe). The prototypes were powered by un-reheated Armstrong Siddeley Sapphire turbojets, as the selected Rolls-Royce Avon engines which powered subsequent production aircraft had fallen behind schedule due to their own development problems. Due to the limited internal space of the fuselage the fuel capacity was relatively small, giving the prototypes an extremely limited endurance, and the narrow tyres housed in the thin wings rapidly wore out. Outwardly, the prototypes looked very much like the production series, but they were distinguished by the rounded-triangular intakes, short fins and lack of operational equipment.