The UK Airprox Board investigated seven incidents involving drones, four of which were classified as being in the most serious bracket.

The Guardian reports that pilots believe that a collision with an airliner could be catastrophic, and that the impact of a drone strike on a light plane or helicopter would almost certainly bring those aircraft down.

Steve Landells, a flight safety specialist at the British Airline Pilots Association (Balpa), is quoted in the paper: “The reports that UK Airprox gets are the ones that are seen. But when you’re flying at more than 100mph, the chances of seeing a typical, 18-inch wide drone are small. We don’t know if this is the tip of the iceberg. With the massive increase in drone sales, we fear we might see a dramatic rise in close calls.”

A major incident in the holding pattern around Heathrow at 8,000ft was particularly concerning, he said: “That means the problem is getting higher and drones are getting into more conflict.”

Since drones fly with hard, lithium ion batteries, if one hits a jet engine, it could potentially cause an “uncontained engine failure, with bits of metal flying off penetrating cabin and fuel tanks”.

The UK does not currently have a mandated registry of drone pilots or UAVs which means that many consumer drones are being flown without proper checks. The growing number of incidents could lead to regulations being tightened.