A Porsche testdriver is dead following an accident involving a prototype Porsche 911. Investigators are unsure what caused the accident.

A 51-year-old died in Germany this morning following an accident involving a Porsche 911 cabrio prototype. The victim, a Porsche engineer, was driving the car on the A5 Autobahn when he lost control and slammed into a guard rail.

The one-car accident happened at 2:45am just past the Weiterstadt exit on the motorway. Photos of the scene show the sports car wedged under the barrier, with the rail making contact near where the driver's head and torso would have been.

A spokesman for Porsche said the engineer "was one of the most experienced test drivers." The company representative confirmed that the car was a camouflaged cabrio covered with foil to mask parts of the vehicle's appearance. He also noted that, although it did not rollover, the convertible does not have the same resilience as a coupe.

Police and fire department personnel responded to the scene from nearby Darmstadt. The public prosecutor there has taken charge of the ongoing investigation. The location of the accident was midway between Frankfurt and Heidelberg, in the German province of Hessen.

It is not yet known what caused the accident. Witnesses say the driver, who has worked for the German company for 25 years, was driving a prototype of a new Porsche 991 model at a high rate of speed. That stretch of road does not have a speed limit. He was being followed by a second Porsche testdriver in a camouflaged four-door Porsche Panamera prototype.

That driver successfully evaded the crash, and escaped unharmed. Although the accident did take place at an unusual hour, both drivers were working on a 10pm to 6am shift. An initial damage estimate is said to be several thousands of Euro.