A drone the size of a football was flown as close as 20 metres from a passenger jet, according to an official investigation.

The Airbus A320's first officer shouted "look!" as the device passed next to the right wing at around 11,000ft above south-east London, the UK Airprox Board (UKAB) said.

A minority of drone operators are "flagrantly disregarding regulation and common sense" in pursuit of "ever more spectacular video footage", the board warned.

The incident involving the A320 was assessed as having the most serious degree of risk.

There was insufficient time for the flight crew to take action to avoid a potential collision with the drone, which had a flashing magenta light.

It was at the same altitude and passed just 20 to 40 metres from the aircraft.

Air traffic control was immediately alerted to the near miss which occurred at 6.05pm on August 4, when the airliner was in a holding pattern over Biggin Hill used for flights approaching Heathrow Airport.

Details were passed to the Metropolitan Police but the drone operator could not be traced.

The UKAB concluded that the person flying the drone had endangered the A320 and its occupants while flying in restricted airspace without permission.

Civil Aviation Authority rules also state that drones should not be flown above 400 feet.

A collision "had only been narrowly avoided" and chance had played a "major part", the report noted.

The incident was one of three near misses involving drones which were investigated by the UKAB at its latest meeting.

On July 20 an Embraer 190 jet was in a climbing turn at 2,700 feet over the Queen Elizabeth Olympic Park in Stratford, east London, shortly after taking off from London City Airport when a drone passed just nine metres above it and 20 metres to its left.

In another case on the same day, a sharp-edged bright yellow drone with a width of 60cm was flown at "exactly the level of the flight deck window" of a Boeing 767 aircraft approaching Manchester Airport.

The report found that the device was "so close it must have passed over the right wing".

In both examples the drone operators were not found and the incidents were recorded as having the highest level of risk.

Last month the Civil Aviation Authority launched a website to promote its code of conduct for drone users - named the Dronecode - which has been revised in a bid to make it easier to understand.

A spokesman for the regulator said: "It is totally unacceptable to fly drones close to aircraft and airports. Anyone flouting the rules can face severe penalties including up to five years in prison."