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.
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.
Flight tracking website Flightradar24.com showed many aircraft circling around the west London airport.
ITV reporter Sarah Rodgers is also among those currently grounded at Heathrow.
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