Tesla is recalling more than two million cars after the US regulator found its driver assistance system, Autopilot, was partly defective.

 It follows a two-year investigation into crashes at the firm owned by billionaire Elon Musk, which occurred when Autopilot was in use.

Tesla said it would issue a software update “over the air” to fix the issue.

The recall applies to almost every Tesla sold in the US since the Autopilot feature was launched in 2015.

Autopilot is meant to help drivers with steering, acceleration and braking – but despite the name it critically cannot be driven without user input.

The US National Highway Traffic Safety Administration (NHTSA) said the recall was due to an issue with Autopilot’s driver monitoring system, which detects whether the driver is paying attention.

This is meant to check things such as whether the driver has their hands on the wheel.

But Tesla said the system’s controls “may not be sufficient to prevent driver misuse”.

The NHTSA said it would continue to monitor the software once it is updated.

It comes a week after a former Tesla employee told journalists he believed the technology was not safe enough to be used on public roads.

It is the second recall this year affecting Tesla vehicles.

Tesla is yet to comment.