Operational Requirement F.155
Operational Requirement F.155 was a specification issued by the British Ministry of Supply on 15 January 1955 for an interceptor aircraft to defend the United Kingdom from Soviet high-flying nuclear-armed supersonic bombers. Discussion about the need for a new supersonic interceptor had been ongoing for some time in the early 1950s, and several designs introduced, but improving radar systems and weapons mooted the need for better aircraft in the short term.
Source: Wikipedia — Operational Requirement F.155 (CC BY-SA 4.0)