Upon announcing the closure of Heathrow, an airport spokesman said the airport was working with the Metropolitan Police and had stopped all flights as "a precaution".
Flights have resumed at Heathrow Airport after a brief suspension amid reports of a drone sighting. "We apologise to passengers for any inconvenience this may cause", it said.
Gatwick said it had spent £5m to prevent future attacks.
A Heathrow spokesperson said they were working with police to "prevent any threat to operational safety".
Heathrow serves more than 200,000 people every day.
ITV reporter Sarah Rodgers was among those now grounded at Heathrow.
A drone flies in Hanworth Park in west London, as a British Airways 747 plane prepares to land at Heathrow Airport behind. "Like WTAF?!", one wrote.
Nadim Baig said it was just his "luck" to have his flight grounded before it was due to take off. Drones were reportedly spotted at the airport 67 times and authorities detained and released two suspects following investigations.
Mr Grayling said: "We are in contact with Heathrow Airport concerning the drone sighting. Would seem that I should expect 2-3 days wait?"
And a fourth said: "Sitting on The runway at London's #Heathrow Airport has been closed due to drone activity".
She told the BBC: "At the moment, we've been told we're not going anywhere".
This incident comes after operations at Gatwick Airport, about 30 miles southeast of Heathrow, were brought to a screeching halt amid the busy Christmas travel season because of drone sightings there. More than 140,000 passengers had their flights disrupted and more than 1,000 flights were affected between December 19th and December 21st 2019.
On Monday police was given new powers to deal with drone pilots breaking the law around airports.
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