London’s Heathrow Airport, Europe’s busiest hub in terms of passenger numbers, said it has suspended all departing flights on Tuesday following a drone sighting.

“We are responding to a drone sighting at Heathrow and are working closely with the Met Police to prevent any threat to operational safety,” the airport’s Twitter account said.

“As a precautionary measure, we have stopped departures while we investigate. We apologise to passengers for any inconvenience this may cause,” it said.

Arriving planes, however, are continuing to land at Heathrow.

The incident comes just over two weeks after drone sightings at Gatwick, Britain’s second-biggest hub, caused travel misery for tens of thousands of people after flights were suspended over three days.

Gatwick has since said it has invested in anti-drone technology, while Heathrow said that it would do so.

In response to the chaos at Gatwick, Transport Secretary Chris Grayling on Monday told parliament that drone exclusion zones around British airports were being extended and operators will have to register.

Police will also be allowed to fine users up to £100 (112 euros, $128) for failing to comply when instructed to land a drone, or not showing registration to operate a drone.

Grayling said the disruption at Gatwick between December 19 and 21 was “deliberate, irresponsible and calculated, as well as illegal”.

The exclusion zone around airports is currently one kilometre (half a mile) and this will be extended to five kilometres.

From November 30 this year, operators of drones weighing between 250g and 20kg will also have to register and take an online pilot competency test.