Work on two prototypes was authorised in November 1943, but proposals for alternative engines delayed progress. In 1945, it was decided that as well as adding another Centaurus-engined prototype, there should be three prototypes with the Napier E.122 (a development of the Sabre) as Specification S.10/45. The Ministry believed that this would enable Blackburn to develop their knowledge of aerodynamic and structural design and support the engine development at Napier. However, it was found that the S.10/45 aircraft could only be balanced if the E.122 powerplant was placed behind the pilot. The necessary redesign and weight increase, coupled with the limited funds available to the Royal Navy, meant that it could no longer be justified and the S.10/45 was cancelled on 8 October. While in final design, the Centaurus 77 engine with contra-rotating propellers was cancelled in January 1946 and a conventional Centaurus 57 was substituted. This engine was found to require flexible mounts and was modified into the Centaurus 59. The vertical stabiliser and rudder had to be enlarged from to to counteract the new engine's torque. In September 1946 a strength analysis conducted by the Ministry of Supply revealed that the aircraft would require strengthening to serve as a strike fighter and that a costly redesign would be required to bring it up to requirements, making it comparable in weight and performance to the Westland Wyvern which had already flown so no contract was placed for production aircraft.