The Army was called in as Heathrow suspended all departures on Tuesday due to reported drone sightings just weeks after a similar debacle caused chaos at Gatwick.

The London airport halted all outbound flights at around 5pm as a "precautionary measure" to "prevent any threat to operational safety".

They were given the all clear around an hour later, as staff said they would continue to monitor the situation and government sources said they remained on standby.

The military was brought in after police officers reported seeing a drone at Heathrow and a spokesman for the Ministry of Defence said: "We are deploying specialist equipment to Heathrow Airport at the request of the Metropolitan Police."

Commander Stuart Cundy of the Met said: "Police officers were amongst those who saw the drone and a full criminal investigation has been launched.

"We are carrying out extensive searches around the Heathrow area to identify any people who may be responsible for the operation of the drone," he said.

"I want to be clear that the illegal operation of drones at an airfield is extremely dangerous. "Under the Aviation Security Act it is an offence to endanger the safety of an aircraft, anyone found guilty of this offence could face a life sentence.

"We are deploying significant resources - both in terms of officers and equipment - to monitor the airspace around Heathrow and to quickly detect and disrupt any illegal drone activity; some of which are as a result of learning from the incidents at Gatwick.

"Following today's sighting, military assistance has been implemented to support us. However, we will not be discussing in any further detail the range of tactics available to us as this would only serve to potentially undermine their effectiveness."