The Civil Aviation Authority (CAA) says that parcel deliveries using drones in the U.S. will be allowed only after new safety crash-prevention technology is developed.

A document released by CAA says that drones will need specialist software making them visible planes, air controllers and each other before they can be flown beyond the sight of human operators.

The regulator has also said delivery and commercial drones will need automated crash avoidance capabilities as standard, so they can detect and get out of the way of fast oncoming objects like planes and gliders.

Currently, drones are not allowed to be flown more than 500 meters away from the human behind the controls, unless by the emergency services in rare and exceptional circumstances. Drones can only be flown beyond the line of sight when the CAA gives specific permission, such as for commercial tests, and even then in designated temporary no fly zones.

Amazon carried out its first successful UK trials of drone parcel deliveries in 2016.

In the U.S., companies including Amazon, Fedex and UPS have started parcel delivery services using drones.