Mercedes-AMG GT four-door - 800bhp hybrid system under development

This, the Mercedes-AMG GT four-door, will become the brand's most potent series production model when it goes on sale in September 2018.

Available in pure combustion engine or top-spec hybrid form, the future Porsche Panamera rival is predicted to have up to 800bhp, making it about 223bhp more potent than AMG GT R. Only the 1000bhp Project One will beat it for output, but that car will be a limited-run model.

The GT four-door is currently being put through its paces on road and circuit, where hardware and software settings are being honed and tested to the limit in the run up to its market launch in just over a year. 

The latest sighting shows that its design will take heavy influence from the GT concept of this year's Geneva motor show, with LED headlights that are clearly inspired by the concept's, as well as a similarly slim glass line. At the back, heavy cladding covers the tailgate’s lines, but it’s likely that the same swooping lid leads into thin taillights that resemble those of the GT coupé.

Despite the link to the GT sports car in its name, the four-door is actually more closely related to the E-Class. It sits upon a heavily reworked version of that car's structure with a stretched wheelbase. Like the GT concept, the body is expected to be longer, wider and shorter than the E-Class, giving the car a more sporting stance. 

The concept was 5065mm long, 2046mm wide and 1405mm high, making it larger and lower than the Panamera – emphasising the car’s aggressive design. The car’s drivetrain layout will also be closely related to the E-Class, meaning it will locate its gearbox up front, rather than in a rear-mounted transaxle position like the GT sports car.

At first, the four-seat GT will be offered with a twin-turbocharged 4.0-litre V8 petrol engine, but it is not yet known if it will be the dry-sump M178 unit of the GT or the wet-sump M177 used in the AMG E 63.

Drive will be sent to all four wheels and power is expected to exceed 600bhp, ranking the four-door’s unit above even that of the hardcore GT R track-focused model. But this output will be eclipsed in 2019 when a hybrid version is launched. 

That car’s drivetrain was previewed in the concept which mated an electric motor to AMG’s 4.0-litre V8 to produce 805bhp and enable a sub-3sec 0-62mph time. Mercedes Formula 1 battery technology was used to warrant recharging through brake energy recuperation and input from the combustion engine.

Mercedes may not name its four-door GT model the GT 4, because it already produces a GT4 racing car that’s based on the GT road car. However, GT could be used in some way to distance the model from the four-door CLE, which will replace the CLS, to emphasise the car’s powerful performance.

When order books open, the GT four-door will likely be priced slightly higher than the 542bhp Porsche Panamera Turbo which starts at £113,075. The hottest hybrid version could command significantly more.