Heathrow has resumed flights after departures were disrupted for an hour on Tuesday following reports of a drone sighting, a spokeswoman for Europe's biggest airport said.
Flights from London's Heathrow airport were halted for about an hour on Tuesday after a drone was sighted near Europe's busiest air hub, raising fears that the chaos that affected rival Gatwick last month could be repeated on an even larger scale.
"We are responding to a drone sighting at Heathrow," the airport said. "As a precautionary measure, we have stopped departures while we investigate. We apologise to passengers for any inconvenience this may cause."
London's Metropolitan Police said they had received reports of a drone near the airport at about 1705 GMT on Tuesday, which they were investigating with airport authorities.
It said police officers were among those who reported seeing a drone and it had launched a full criminal investigation.
The airport confirmed about an hour later that take-offs had resumed. Flight tracker websites showed flights departing from 1811 GMT.
A Reuters witness in a plane on the runway at Heathrow said multiple aircraft were waiting for permission to take off before being later told that things were starting to move.
London's second busiest airport, Gatwick, was severely disrupted when drones were sighted on three consecutive days in December, resulting in about 1,000 flights being cancelled or diverted and affecting 140,000 passengers.
Gatwick's drone nightmare - the most disruptive yet at a major airport - exposed a new vulnerability that will be scrutinised by security forces, airlines and airport operators across the world.
The British army was forced to deploy military technology to guard the area around Gatwick, reassuring the airport that it was safe enough to fly.
Flights at Heathrow are operating normally. Our focus is always the safety of our passengers and colleagues and we apologise to those passengers whose journeys have been unfairly affected this evening.— Heathrow Airport (@HeathrowAirport) January 8, 2019
Police said they were carrying out extensive searches around the Heathrow area to identify any people who may be responsible for the operation of the drone.
"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," police commander Stuart Cundy said in a statement.
He said military assistance has been implemented after the sighting but he would not give any further details as it could undermine the effectiveness of the measures.
British Transport Secretary Chris Grayling said he had been in contact with Heathrow Airport concerning the drone sighting.
"I have already spoken to both the Home Secretary and Defence Secretary and the military are preparing to deploy the equipment used at Gatwick at Heathrow quickly should it prove necessary," he said.
Both airports responded to the Gatwick incident by ordering military-grade anti-drone technology.
Heathrow said it was working closely with authorities including the police and looking at relevant technology to combat the threat of drones.
The airport handled 78 million passengers in 2017 and is the main hub for British Airways. Its busiest routes in terms of passenger numbers are to Dubai and New York-JFK.