The US army is testing small helicopter-style machines that could hover and be recharged at 500 metresGETTY IMAGES

Military researchers are developing drones that might never need to land to refuel as they keep watch over troops and carry out surveillance.

The system involves firing a laser at a photovoltaic cell — a small solar panel — fixed to the drone, which converts the beam into electrical power. The US army is testing the concept on small helicopter-style flying machines that could hover above a base or convoy indefinitely and be recharged at a distance of 500 metres.

The Defence Advanced Research Projects Agency the arm of the US military that pioneered the development of the internet, will soon explore the feasibility of using lasers to power larger unmanned aircraft over distances of up to seven miles.

A similar mechanism has been developed…