Departures have been suspended at Heathrow after a suspected drone sighting.
The west London airport said it is working with the police after the device was spotted.
A Heathrow spokesman said: “We are responding to a drone sighting at Heathrow and are working closely with the Metropolitan Police to prevent any threat to operational safety.
“As a precautionary measure, we have stopped departures while we investigate.
“We apologise to passengers for any inconvenience this may cause.”
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.
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.
The Army was brought in to help deal with the travel chaos and was not stood down until after Christmas.
The anti-drone equipment can detect and jam communications between a drone and its operator and was deployed on a roof at Gatwick.
The system, which is said to have a range of several miles, uses four radars to give 360-degree detection in order to identify and track targets.
Following the end to the initial drone-related disturbance at Gatwick, Security Minister Ben Wallace said: “I can say that we are able to now deploy detection systems throughout the UK to combat this threat.”
On Tuesday, the Government announced a package of measures designed to give police extra powers to combat drones.
The exclusion zone around airports will be extended to approximately a 5km-radius (3.1-miles), with additional extensions from runway ends.
Ministers also announced that from November 30 operators of drones, weighing between 250g and 20kg, will be required to register and take an online drone pilot competency test.
Police will also be able to issue fixed-penalty notices for minor drone offences to ensure immediate and effective enforcement of the new rules.
Fines of up to £100 could be given for offences such as failing to comply with a police officer when instructed to land a drone, or not showing their registration to operate a drone.