The US government is investigating reports of Tesla cars braking unexpectedly on motorways.

The so-called "phantom braking" problem is being looked at by US regulator the National Highway Traffic Safety Administration (NHTSA).

It received 354 complaints in the past nine months and its investigation will cover approximately 416,000 Tesla Model 3 and Model Y vehicles from 2021-22.

Drivers say the issue occurs using the Autopilot driver assistance system.

The feature gives the vehicle control over some elements of braking and steering when driving, although it is not a substitute for a human driver.

Despite the name, Tesla recommends drivers remain vigilant and supervise their vehicle, noting the Autopilot ADAS system "does not make the vehicle autonomous".

Tesla is currently under investigation by the NHTSA over two other matters.

In December 2021, it disabled its Passenger Play feature that allowed games to be played on its touchscreen while the car is in motion, leading to an open investigation covering an estimated 580,000 vehicles.

And last August, the NHTSA started to look into the role of the Autopilot system in 11 crashes involving emergency vehicles, covering approximately 765,000 Tesla cars.

The NHTSA's Office of Defects Investigation (ODI) has begun a "preliminary evaluation" into Tesla over the complaints. This is the stage before the agency could officially issue a recall of the vehicles.

It says there have been no crashes, injuries or fatalities as a result of the incidents.

"The complaints allege that while utilising the ADAS features including adaptive cruise control, the vehicle unexpectedly applies its brakes while driving at highway speeds," the report says.

"Complainants report that the rapid deceleration can occur without warning, at random, and often repeatedly in a single drive cycle.

"ODI is opening this preliminary evaluation to determine the scope and severity of the potential problem and to fully assess the potential safety-related issues."

The NHTSA makes customer complaints publicly available on its website, so consumers can compare vehicle safety.

In one complaint from 11 February 2022, the driver says: "Heavy braking occurs for no apparent reason and with no warning, resulting in several near misses for rear-end collisions... this issue has occurred dozens of times during my five months and 10,000-mile ownership."

In another dated 3 February 2022, the user complains of "phantom braking for no apparent reason", stating that their car suddenly decelerated from 73mph down to 59mph "in two seconds".

The BBC has approached Tesla for comment.