A man has died in the first known fatal crash involving a car in self-driving mode, prompting an urgent investigation by electric car makers Tesla and US authorities.

Joshua Brown, 40, died when his Tesla Model S collided with the white trailer of a lorry on a dry Florida road in May.

According to the Florida Highway Patrol report, the Tesla's windscreen hit the bottom of the trailer as it passed underneath, and the car kept going, leaving the road.

It continued, striking a fence, crossing a field, passing through another fence before finally hitting a pole about 30m south of the road.

The US National Highway Traffic Safety Administration (NHTSA) have begun an investigation into the autopilot system used in 25,000 Model S cars.

The NHTSA said preliminary reports indicate the crash occurred when a tractor-trailer made a left turn in front of the Tesla at an intersection.

Tesla said in a blogpost: "Neither Autopilot nor the driver noticed the white side of the tractor trailer against a brightly lit sky, so the brake was not applied."

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The luxury electric car explained customers are required to give "explicit acknowledgement" that they realise the Autopilot feature is new technology still under development, otherwise the system will remain off.

Tesla added: "When drivers activate Autopilot, the acknowledgment box explains, among other things, that Autopilot 'is an assist feature that requires you to keep your hands on the steering wheel at all times,' and that 'you need to maintain control and responsibility for your vehicle' while using it."

The firm says Autopilot "results in a statistically significant improvement in safety".

The NHTSA said the crash calls for "an examination of the design and performance of any driving aids in use at the time of the crash."

A Tesla Model S charges at a Tesla Supercharger station (Reuters)

The federal agency said it has opened a preliminary investigation, which is the first step before it can seek to order a recall if it finds the vehicles to be unsafe.