Authorities in the US state of Florida are investigating the death of a driver who was using a self-driving car.

Reports say Joshua Brown died after a tractor trailer drove out in front of his Tesla vehicle.

It is thought the car failed to detect the white trailer against a bright sky.

Tesla says it is the first fatality involving its autopilot system, in more than 130 million miles of use.

In a statement, the car-maker says: "What we know is that the vehicle was on a divided highway with autopilot engaged when a tractor trailer drove across the highway perpendicular to the Model S."

"Neither autopilot nor the driver noticed the white side of the tractor trailer against a brightly lit sky, so the brake was not applied."

"The high ride height of the trailer combined with its positioning across the road and the extremely rare circumstances of the impact caused the Model S to pass under the trailer, with the bottom of the trailer impacting the windshield of the Model S."

Tesla adds that it is "important" to remember the system is new technology, and still in a public beta phase before it can be enabled.

It says that when drivers activate autopilot, an acknowledgment box explains that it is an "assist feature" that requires drivers to keep their hands on the steering wheel at all times - and that "you need to maintain control and responsibility for your vehicle."

The company concludes saying they are "beyond saddened" by the man's death, extending sympathies to his family and friends.