Staff Writer (Updated: 5/11/2016):
In 1962, the British Royal Air Force and the French Air Force found themselves with a similar need for a new capable aircraft system. The British sought to replace their aging series of Folland Gnat T.Mk 1 and Hawker Hunter T.Mk 7 trainers with a modern advanced supersonic type while the French were looking for an intermediate subsonic aircraft type to replace their Fouga Magister and Lockheed T-33 jet trainers, their Dassault Mystere IV fighters and fill the gap behind their Mirage family of high performance fighters. In 1965, the two nations formally came together with an agreement and, in 1966, the two sides were represented by the British Aircraft Corporation (Warton Division) and Breguet. The collaborative effort was given the acronym of "SEPECAT" - "Societe Europeenne de Production de l'Avion d'Ecole de Combat et d'Appui Tactique" which translated to "European Production Company for the Combat Training and Tactical Support Aircraft". To showcase Breguet's lead in the design effort, BAC registered its company within France. The joint effort would become the first time that two major European nations would attempt to produce an operational combat aircraft jointly.
As the project gained steam, the Royal Air Force revised its need for an advanced two-seat jet trainer while the French Air Force was seeking a dual-role solution covering a ground-attack strike fighter with excellent Short Take-Off and Landing (STOL) capabilities as well as an advanced jet trainer. The need for both sides was further refined when both parties dropped their two-seat advanced trainer requirement and concentrated efforts on a close-support, single-seat strike and interdiction platform. The trainer requirement had indeed been filled by acceptance of the Hawker Siddeley Hawk for the British and the Alpha Jet for the French. The Royal Air Force now looked to replace their fleet of American-made McDonnell Douglas F-4 Phantom IIs (FG.R Mk 2 in RAF service) and drew up plans for a new aircraft to cover reconnaissance, strike and close-air support sorties. The French looked to replace their aged Dassault Etendard IV carrier aircraft. In addition to a single-seat, multi-role fighter design, a two-seat variant was envisioned to facilitate pilot conversion training to the new mount. Profits from the sale of the aircraft would be split 50/50 between BAC and Breguet. Several designs from both parties were researched before the sides settled on the Breguet Br.121 concept with the wings and high lift elements designed by BAC. The new aircraft was christened the "Jaguar".
As with the airframe, the powerplant would also be designed jointly by the British firm of Rolls-Royce and the French firm of Turbomeca as the "Adour" series. This new turbofan engine would be designed with afterburner capability to help make the aircraft supersonic in nature. In its early stages, a complicated variable-geometry air intake was entertained to help optimize engine efficiency at speeds in excess of Mach 1.0 but this thinking later gave way to a more conventional fixed intake design to help move series production along.