"As a precautionary measure, we have stopped departures while we investigate", it said.
Heathrow officials said shortly before 6 p.m. Tuesday that flight departures have been suspended as a precaution. "We apologise to passengers for any inconvenience".
Gatwick said it had spent £5m to prevent future attacks.
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".
"I have already spoken to both the Home Secretary and Defence Secretary and the military are preparing to deploy the equipment used at Gatwick at Heathrow quickly should it prove necessary".
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John Stewart, of the Heathrow noise reduction campaigning group Hacan, said: "Although a third runway will clearly bring many losers, one long-lasting impact of these proposed changes is that a break from aircraft noise would be guaranteed to many more people than who now enjoy it, as Heathrow would make the biggest changes to its flight paths since opening in 1946".
Heathrow Airport has confirmed that it has grounded all departing flights while it investigates the situation.
Scotland Yard said they received reports of a drone "in the vicinity of Heathrow Airport" just after 5pm. "Like WTAF?!", one wrote.
Arriving planes, however, are continuing to land at Heathrow.
An extra 25,000 flights a year could come through Heathrow before the third runway is built, in plans revealed by the airport as it launched a fresh consultation over its airspace and operations. No arrivals, no departures due to Drone activity in the area'.
Student Gabriella Linning, 20, said she is on a grounded flight to Newcastle, told MailOnline the pilot of her plane said passengers should let staff know if they wish to disembark.
The news comes weeks after drone sightings were reported between December 19 to 21 at Gatwick, the UK's second-busiest airport, amid holiday chaos just before Christmas.