The first fully road legal prototype of Porsche’s upcoming 325kph petrol-electric hybrid powered 918 Spyder has been spied undergoing shakedown tests near Porsche’s Weissach R&D test facility.

Sporting a look highly reminiscent of the concept first displayed at the 2010 Geneva motor show, the new two-seater is set to crown the Porsche line-up when it is launched next year at a price already confirmed at £621,600 at current exchange rates.

But while it appears similar to the concept, close inspection reveals a number of changes have taken place in the production process. Among them is a smoother looking front end with smaller air ducts out on the front corners and a bonnet that does without the air vent of show car.

The windscreen also sits at a more upright angle and the cabin has been extended slightly in length – as evidenced by the prototype’s longer doors.

At the rear, there are more prominent hunches over the rear wheels and a longer rear overhang – presumably a requirement to see the new car house its advanced petrol-electric driveline, which is concentrated wholly within the lower half in a move Porsche says is aimed at providing the lowest possible centre of gravity.

While appearing like a coupe, the 918 Spyder will, as its name suggest, be an open top. The carbonfibre roof panel, divided in two parts, detaches.

Also clearly visible on the prototype pictured here is the 918 Spyder’s so-called top pipes – exhausts that exit out of the top of the engine bay just behind the monocoque’s integral carbonfibre roll over hoops.

The towing rear wing worn by the car shown has been created to replicate the downforce provided by the production car’s more complex multi-stage retractable device.

Power comes from a 4.6-litre V8 mated to a pair of electric motors: one mounted on the front axle to drive the front wheels and another in the gearbox housing to drive the rear wheels.

Total power is 762bhp, with Porsche claiming 0-100kph performance of just 2.8sec.

Production will be limited to 918 cars with the first example set to be revealed on 18 September - 9/18 as it is known in some parts of the world. Assembly will take place on a dedicated line at Porsche’s traditional Zuffenhausen headquarters in Stuttgart.

More pictures here