An airport spokesperson said they were working with the Metropolitan Police earlier and had stopped departing flights "as a precaution".
"As a precautionary measure, we have stopped departures while we investigate".
In a statement shared on Twitter by the Department for Transport, he said: "We are in contact with @HeathrowAirport concerning the drone sighting". She said: "It is crucial that our plans maximise the benefits of expansion across the country, including for the communities closest to us".
ITV reporter Sarah Rodgers was among those now grounded at Heathrow.
'We apologise to passengers for any inconvenience this may cause'.
ITV reporter Sarah Rogers said her plane's pilot grounded her flight to Manchester while police "investigate possible drone activity".
Heathrow and Gatwick responded by ordering military-grade anti-drone defences worth "several million pounds".
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A drone flies in Hanworth Park in west London, as a British Airways 747 plane prepares to land at Heathrow Airport behind.
One tweeted: "We've been sitting on the runway at Heathrow for like an hour and a half". No arrivals, no departures due to Drone activity in the area'.
Nadim Baig said it was just his "luck" to have his flight grounded before it was due to take off.
BBC producer Kathy Long, speaking from a plane waiting to depart at Heathrow, confirmed passengers were told they won't be departing in the near future.
"We are committed to delivering expansion responsibly, and we encourage everyone to have their say and take part". More than 140,000 passengers had their flights disrupted and more than 1,000 flights were affected between December 19th and December 21st 2019.
Whoever had their fun with a drone at Gatwick is about to hand loads more anti-drone powers to the police, as the Home Office has announced the results of a consultation into What To Do About Drones.
The exclusion zone around airports will be extended to approximately a five kilometre radius, with additional extensions from the end of runways.