A passenger jet was involved in a near-miss with a drone at London's Heathrow Airport.
The drone was involved in an incident with an Airbus A320, which can carry up to 180 passengers, on 22 July while flying at an altitude of 213 metres.
An official report into the incident will be published next Friday.
The report comes amid growing concern over the risks of shop-bought drones to commercial flights.
The incident is expected to be given an incident risk rating of A, the highest of five categories as defined by the UK Airprox Board.
This is defined to mean there was a "serious risk of collision".
Drones are set to be one of this year's biggest-selling Christmas presents, according to electronics retailers, with some models available for as little as €38.
The British Airline Pilots Association says drones which share airspace with passenger and freight planes should be flown only by operators with training equivalent to a pilot's.
The Civil Aviation Authority, which regulates airspace in Britain, stipulates that operators of drones used for collecting data or surveillance must obtain permission before flying in a congested area or near people and property.
Other types of drones which weigh less than 20kg are exempt from most aircraft regulations.