Departures at Heathrow have been stopped after a drone was sighted, the airport says.
A Heathrow spokeswoman said the airport was 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.
Flights appeared to be taking off again about half an hour after Heathrow issued the statement, according to the Flightradar24 website.
The Metropolitan Police said they received reports of a drone sighting near Heathrow at about 17:05 GMT.
“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.
BBC producer Kathy Long, speaking from a plane waiting to depart at Heathrow, said passengers had not been told officially what was behind the delay.
“At the moment, we’ve been told we’re not going anywhere,” she said.
More than 140,000 passengers at Gatwick were affected during 36 hours of chaos between 19 and 21 December.
About 1,000 flights were cancelled there over three days due to the drone sightings.
Gatwick said it had spent £5m to prevent future attacks. Heathrow also confirmed it would be buying systems to guard against drones.
Passengers stuck at Heathrow expressed their frustration at having to wait to depart while the airport responded to the sighting.