Heathrow said in a statement: "Based on standard operating procedures, working with Air Traffic Control and the Met Police, we have resumed departures out of Heathrow following a short suspension".
"We've been sitting on the runway at Heathrow for like an hour and a half", another person said.
Flights for more than 120,000 people were disrupted at Gatwick before the Christmas holiday, triggering the deployment of the military to monitor the skies.
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One user, called Nadim Baig, said: 'My luck, drone sighting and now my flight is at a standstill just before departure'.
Police and airport officials were investigating the reported sighting, which was not confirmed.
"As a precautionary measure, Heathrow Airport has stopped departures and officers based at Heathrow are now investigating the reports with colleagues from Heathrow Airport", they said.
Gatwick's drone nightmare - the most disruptive yet at a major airport - exposed a new vulnerability that will be scrutinised by security forces, airlines and airport operators across the world.
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BBC producer Kathy Long, speaking from a plane waiting to depart at Heathrow, said passengers had not been told officially what was behind the delay. The latest sightings come a month after a drone caused major travel disruptions in December for almost 150,000 travellers.
The alleged sighting came four days after both Heathrow and Gatwick airports reported they were investing millions of pounds in equipment to prevent future flight disruption.
Transport Minster Chris Grayling said the military are "preparing to deploy equipment used at Gatwick should it prove necessary". It hosts 81 airlines that fly to 204 destinations in 85 countries, and its most popular destination is NY.
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