Plane manufacturer Boeing has issued an update to the control systems on the 737 Max aircraft, two of which have crashed in the past five months.

The planes are currently grounded following the incidents, in Indonesia and Ethiopia.

Investigators have not yet determined the cause of the accidents, and it remains unclear when the aircraft might return to service.

As part of the upgrade, Boeing will install as standard a warning system which was previously an optional safety feature.

Neither of the planes, operated by Lion Air and Ethiopian Airlines, that were involved in the fatal crashes carried the alert systems.

It is designed to warn pilots when sensors produce contradictory readings.

Boeing said airlines would not be charged extra for that safety system to be installed.

The plane maker has also issued an upgrade to the Manoeuvring Characteristics Augmentation System, a mechanism that has been linked to the crashes.

Boeing has redesigned the software so that it will disable MCAS if it receives conflicting data from its sensors.

Announcing the package of upgrades, Boeing said a final version of the software would be submitted to the Federal Aviation Authority by the end of the week.

However, airlines will have to install the new software, give feedback on its performance, and train pilots before the changes could be certified and the planes passed safe to fly again.