Heathrow Airport has confirmed that it has grounded all departing flights while it investigates the situation.
A Heathrow spokeswoman said the airport was working with police to "prevent any threat to operational safety".
Another wrote; "Grounded at #Heathrow whilst waiting to fly home to Manchester, first officer says flights suspended whilst police investigate possible #drone activity". "We apologies to passengers for any inconvenience this may cause".
Scotland Yard said it "received reports of a sighting of a drone in the vicinity of Heathrow airport" at about 5.05pm.
'As a precautionary measure, Heathrow Airport has stopped departures and officers based at Heathrow are now investigating the reports with colleagues from Heathrow Airport, ' the force added.
Between December 19 and 21, Gatwick was repeatedly forced to close due to reported drone sightings, causing mass disruption to passengers, with about 1,000 flights affected.
Gatwick was severely disrupted shortly before Christmas when drones were sighted on three consecutive days.
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Transport Minster Chris Grayling said the military are "preparing to deploy equipment used at Gatwick should it prove necessary".
Heathrow Airport has stopped departures after a drone was spotted.
"We've been sitting on the runway at Heathrow for like an hour and a half", another person said.
The government is moving forward with plans to give police new powers to tackle drones misuse and abuse, with the publication of their response to the drones consultation today (7 January 2019). Exclusion zones are now one kilometre around airports and will be extended to five kilometres.
Police could also fine drone users failing to comply with authorities or present registration to operate a drone up to £100.
The government also announced from November, operators of drones weighing between 250g and 20kg would be required to register and take an online drone pilot competency test.