In the mid-1950s Government Defence Policy was to maintain a fleet of four active aircraft carriers to meet both NATO and Imperial commitments. In practice this meant a total fleet of five to six ships, allowing for refits, modernisation and training. The rapid advances in naval aviation technology during this period, such as the angled deck, steam catapults and mirror landing sight, coupled with the planned introduction of the new, heavier second-generation jet fighters such as the Supermarine Scimitar and De Havilland Sea Vixen, rendered the existing Carrier fleet obsolescent. Only (completed in 1955), the rebuilt , and under-construction , due to join the fleet in 1958 and 1959 respectively, had the capability to operate the new aircraft. Thus it was decided to rebuild from 1959 to 1964 to the same standard as "Victorious" and "Hermes", followed by a similar upgrade to "Ark Royal". This left a potential period of five years between 1959 and 1964 when a maximum of three carriers would be available. The Admiralty therefore searched for an interim solution to bridge this feared gap.