Mercedes-AMG GT four-door: leaked image shows 604bhp V8 model

He added that his personal opinion was that the four-door AMG model will be more successful than the company anticipates.

Previewed by the GT concept of 2017, the AMG GT four-door models will come with a range of EQ Power-branded petrol-electric hybrid drivelines featuring differing battery capacities, outputs and electric ranges.

The new Mercedes-AMG model has been conceived as an upmarket replacement for today’s CLS 63 and a rival to the Porsche Panamera. Sales for the production model are scheduled to begin in September 2018.

The car will be launched with the same 604bhp twin-turbocharged 4.0-litre petrol V8, nine-speed automatic gearbox and fully variable four-wheel drive system as the latest E63 S 4Matic, upon which it is largely based. However, Mercedes officials have said there are also concrete plans for it to be sold with hybrid drivelines from 2019 onwards - suggesting the spotted harder version could arrive as part of that line-up.

The decision to develop the car's new hybrid driveline with three differing battery capacities will allow Mercedes-AMG to offer the car with varying power outputs and ranges.

Although the finer details are still under wraps, Autocar has been told that the range-topping hybrid model is set to produce the same 805bhp and offer a similar 31-mile electric range as the GT Concept (pictured above and below).

The other versions under development are said to include a base hybrid driveline with 671bhp and a 19-mile range and a mid-range variant with 738bhp and a 25-mile range.

Sources suggest these units' modular nature will allow them to feature in other AMG models, including the successors to the Mercedes-AMG GT Coupeàand Roadster.

The hybrid driveline for the production version of the GT Concept uses AMG’s twin-turbocharged 4.0-litre V8 engine, mounted longitudinally up front, in combination with an electric motor sited within the rear axle. The engine can send power to all four wheels, while the motor delivers power exclusively to the rear wheels.

AMG says the motor is networked with a combination of extremely light batteries. These have been developed in partnership with High Performance Powertrains, the British-based AMG sister company behind the 1000bhp petrol-electric driveline of the upcoming Project One hypercar.

The operation of the new petrol-electric powerplant has been derived from the hybrid unit used by the Mercedes-Benz Formula 1 team, with the battery charged while the car is being driven through brake energy recuperation and the aid of the petrol engine.