Triumph had once accomplished the transformation from an open roadster to fastback coupe with the Triumph Spitfire and GT6 and management was keen to see whether lightning could strike twice with the conversion of the luxurious Stag to an enclosed coupe. The Triumph Stag Fastback was originally the idea of Spen King, who had taken over as the Chief Engineer from Harry Webster, the plan was to send the original prototype back to Michelotti for conversion into a fastback design study.
First proposed in 1968, the first attempt was rejected by Triumph’s design staff and Michelotti finally produced an acceptable prototype the following year. A running prototype with a 2.5-liter fuel injected engine and two additional cars were built (the last with V8 power). Unfortunately, problems with getting the original Stag to market meant that the project was placed on the back burner but the plan was revived at the end of 1970 by which time the car’s styling had been rationalized with the GT6 Mk III.