Nine evaluation aircraft were ordered as the , an improved version of the P.1127, the first flying on 7 March 1964. The Kestrel had fully swept wings and a larger tail than the early P.1127s, and the fuselage was modified to take the larger 15,000 lbf (85 kN) Pegasus 5 engine as in the P.1127/Kestrel prototype . Due to interest from the U.S. and Germany, the Tri-partite Evaluation Squadron (TES) was formed on 15 October 1964 at , staffed by military test pilots from Britain, the United States and West Germany.
The first three P.1127s crashed, the second and third during development. The first prototype () crashed at the in 1963, caused by a speck of dirt in the air feed lines of the nozzle control motor causing the engine nozzles to stick, but the aircraft was fully repaired and resumed development flying. All the pilots involved survived.
The Hawker P.1127 and the Hawker Siddeley Kestrel FGA.1 were the experimental and development aircraft that led to the , the first () jet fighter-bomber. Kestrel development began in 1957, taking advantage of the 's choice to invest in the creation of the engine. Testing began in July 1960 and by the end of the year the aircraft had achieved both vertical take-off and horizontal flight. The test program also explored the possibility of use upon aircraft carriers, landing on in 1963. The first three aircraft crashed during testing, one at the 1963 .