Departures at Heathrow were temporarily stopped after a drone was reported to have been sighted.
Flights from the west London airport resumed about an hour after police said a drone had been seen.
A Heathrow spokesperson said they were working with police to "prevent any threat to operational safety".
She said: "As a precautionary measure, we have stopped departures while we investigate. We apologise to passengers for any inconvenience this may cause."
It comes after last month's disruption at Gatwick Airport after drones were reported.
The Metropolitan Police said they received reports of a drone sighting near Heathrow at about 17:05 GMT (6.05am NZDT).
"As a precautionary measure, Heathrow Airport has stopped departures and officers based at Heathrow are currently investigating the reports with colleagues from Heathrow Airport," they said.
Earlier, Flight tracking website Flightradar24.com showed many aircraft circling around the west London airport.
The British government yesterday expanded drone-free exclusion zones around British airports from their previous 1km limit, to about 5km.
Heathrow and Gatwick recently also spent millions of pounds on military-grade anti-drone equipment.
Passengers who were stuck at Heathrow expressed their frustration at having to wait to depart while the airport responded to the sighting.
ITV reporter Sarah Rodgers was among those currently grounded at Heathrow.
More to come.