Several passengers stuck at the airport - including those on board aircraft - have also tweeted to confirm that they have been told about the drone sighting.
In a statement, an airport spokesperson said: "We are now responding to a drone sighting at Heathrow and are working closely with the Met Police to prevent any threat to operational safety".
Flight departures have been suspended at London's Heathrow Airport following reports of a drone sighting.
Departures from Heathrow have been suspended after a suspected drone sighting.
Among those grounded at Heathrow was ITV reporter Sarah Rodgers. "We apologise to passengers for any inconvenience".
A Reuters witness in a plane on the runway at Heathrow said multiple aircraft were waiting for permission to take off.
Following the end to the initial drone-related disturbance at Gatwick, Security Minister Ben Wallace said: "I can say that we are able to now deploy detection systems throughout the United Kingdom to combat this threat".
Just before 6pm local-time, a spokesman for Heathrow Airport said they had stopped departures as a precautionary measure.
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Nadim Baig said it was just his "luck" to have his flight grounded before it was due to take off.
York said he is "absolutely certain that there was a drone flying throughout the period that the airport was closed".
Transport Minster Chris Grayling said the military are "preparing to deploy equipment used at Gatwick should it prove necessary".
"At the moment, we've been told we're not going anywhere", she said. About 1,000 flights were cancelled or diverted, affecting 140,000 passengers.
Under new rules announced earlier on Tuesday, drones will have to stay further away from airports.
The British government yesterday expanded drone-free exclusion zones around British airports from their previous 1km limit, to about 5km.