In early 1963 Giovanni Michelotti was commissioned by Standard-Triumph to design a GT version of their recently introduced Spitfire 4 (also designed by Michelotti). An unmodified Spitfire 4 was transported to Michelotti's design studios in Italy and late in 1963 the prototype Spitfire GT4 was returned to England for evaluation.
The styling of the vehicle was a success but unfortunately the extra weight of the GT bodyshell resulted in extremely poor performance, with the 1,147 cc (70 cu in) Spitfire power unit, and plans for producing the Spitfire GT4 were shelved.
However, Michelotti's fastback design for the Spitfire GT4 prototype was (for the 1964 season) adopted by the Triumph racing-programme as it was deemed to provide an aerodynamic benefit over the standard Spitfire body-shape.
Fiberglass copies of the Spitfire GT4's fastback were grafted on to the race-modified Spitfires destined for competition. The Spitfire racing programme was successful and, in 1965, resulted in 13th overall and a 1st in class at the prestigious 24 Hours of Le Mans (beating their main rivals, the MG Midgets). The Spitfire's competitive success and the continuing commercial success of the production vehicle led Triumph to re-evaluate its shelved plans for a GT version of the Spitfire. To overcome the lack of performance inherent in the heavier body-style the Spitfire's 4-cylinder engine was replaced with the more powerful 2-litre (1,998 cc) 6-cylinder engine from the Triumph Vitesse (which shared a similar chassis with the Spitfire and Triumph Herald). The car was further developed and refined and eventually launched as the Triumph GT6 (dropping the "Spitfire" prefix) to identify its GT styling and its 6-cylinder engine.
Contemporary Triumph marketing advertised the GT6 as being developed from the "race winning Le Mans Spitfires" to capitalize on their aesthetic similarities, whereas the Le Mans Spitfires and the GT6 were actually two entirely separate development programmes (the GT programme predating the racing programme).
However, the marketing spin was so successful that many people erroneously believed the Le Mans Spitfires to actually be GT6s.
Source: Wikipedia, 2011