What was the name of the British ballistic missile designed in 1955?

The de Havilland Propellers Blue Streak was a British medium-range ballistic missile (MRBM), and later the first stage of the Europa satellite launch vehicle. Blue Streak was cancelled without entering full production.

The project was intended to maintain an independent British nuclear deterrent, replacing the V bomber fleet which would become obsolete by 1965. The operational requirement for the missile was issued in 1955 and the design was complete by 1957. However, during development it became clear that the missile system was too expensive and too vulnerable to a pre-emptive strike. The missile project was cancelled in 1960, with US-led Skybolt the preferred replacement.

Partly to avoid political embarrassment from the cancellation, the UK Government proposed that the rocket be used as the first stage of a civilian satellite launcher called Black Prince. However, the cost was thought to be too great for the UK alone, and international collaboration was sought. This led to the formation of the European Launcher Development Organisation (ELDO), with Blue Streak used as the first stage of a carrier rocket named Europa.