Gatwick's closure led to more than 100,000 people being stranded or delayed in the worst ever drone-related disruption at an global airport.
It said it is working with police to prevent any threat to operational safety and apologised to passengers "for any inconvenience this may cause".
The groundings come after suspected drone flights caused chaos at Gatwick before Christmas, with tens of thousands of passengers having their travel plans ruined after drones have been flown over one of the country's busiest airports.
Several passengers stuck at the airport - including those on board aircraft - have also tweeted to confirm that they have been told about the drone sighting.
The latest drone-related chaos has already sparked some eye-rolling on social media. Flight tracker websites showed flights departing from 1811 GMT.
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Travel experts estimate that around 40 flights were delayed in the shutdown, and passengers tweeted their frustration at chaos paralysing yet another United Kingdom airport. However, The Met Police are continuing to investigate the circumstances surrounding the drone attack.
One key new power grants the police the option of demanding evidence from drone owners, should the local force think there's "reasonable suspicion" that they were breaking current drone flight rules.
Coincidentally, this comes on the same day the United Kingdom government's Department for Transport announced a series of new drone-related powers for police, including new airport exclusion zones.