The first UK Phantom, a prototype F-4K (designated YF-4K), first flew on 27 June 1966 at the McDonnell Douglas plant in St. Louis. The second made its first flight on 30 August 1966. The two pre-production F-4K aircraft were constructed alongside the prototypes, and were initially used for fit check trials of the various systems to be fitted – the first was used for catapult/arrestor and deck landing trials, and the second was primarily for testing the radar and missile systems. All four were delivered to the UK from 1969 to 1970 for continued test work by the Aeroplane and Armament Experimental Establishment, Ministry of Defence Procurement Executive, Rolls-Royce and British Aerospace. One of the pre-production examples was subsequently delivered to the RAF for operational use, but was lost in 1978. The first F-4M prototype (designated YF-4M) first flew on 17 February 1967, and was also used for fit checks before delivery to the UK. In 1964, 140 new-build Phantoms were ordered for the Fleet Air Arm to serve as the Royal Navy's primary fleet air defence aircraft, with a secondary strike capability. These were procured to replace the Sea Vixen then in service in the role, with the intention that they operate from the decks of four brand new or modernised aircraft carriers. At the time, the Royal Navy's carrier force consisted of five fleet or light fleet carriers of differing sizes and ages. Of the five in service, only and , each displacing approximately 50,000 tons, were big enough to accommodate the Phantom in sufficient numbers, so plans were put in place to rebuild the two ships to enable the operation of the aircraft. At the same time, two new aircraft carriers were planned. The requirements for the intended force of four carriers meant that five squadrons of Phantoms would be needed. However, in its 1966 Defence White Paper, the Government decided to cancel the two new carriers, which led to reductions in the total order for the Royal Navy's Phantoms. Initially 140 were ordered, which was eventually cut to just 48, with options for another seven. The intention was to form a pair of front-line squadrons, each of twelve aircraft, that would operate from the two remaining, heavily modernised fleet carriers. The rest of the aircraft would be a reserve and for training. The Royal Navy received its first F-4K Phantoms, which carried the British designation FG.1, in April 1968. These were assigned to 700P Naval Air Squadron (NAS), which was to serve as the Intensive Flying Trials Unit.