Heathrow staff are working with the Metropolitan Police and all flights have been grounded "as a precaution", an airport spokesperson said.
Outgoing flights were briefly grounded at Britain's largest airport, London Heathrow, following reports of a drone sighting.
Police were called just after 5pm to "reports of a sighting of a drone in the vicinity of Heathrow Airport".
The airport in West London is the businest in the United Kingdom, as well as being the second businest airport in the world in terms of worldwide passenger traffic.
"We've been sitting on the runway at Heathrow for like an hour and a half", another person said.
'As a precautionary measure, Heathrow Airport has stopped departures and officers based at Heathrow are now investigating the reports with colleagues from Heathrow Airport, ' the force added.
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"At the moment, we've been told we're not going anywhere", she said.
Gatwick closed on the evening of December 19 and for much of the day on both December 20 and 21.
The disruption resulted in about 1,000 flights being cancelled or diverting and affecting 140,000 passengers.
Gatwick said it had spent £5m to prevent future attacks.
ITV reporter Sarah Rodgers was among those now grounded at Heathrow. Drones were reportedly spotted at the airport 67 times and authorities detained and released two suspects following investigations. The government will also extend drone exclusion zones around airports.
Fines of up to £100 could be given for offences such as failing to comply with a police officer when instructed to land a drone, or not showing their registration to operate a drone.