Gatwick airport, the UK’s second largest airport, was closed for most of Wednesday night and Thursday morning after it received reports of two drones flying nearby, reports The Guardian. The airport initially had its flights suspended at 9PM on Wednesday evening after the drones were spotted, and although it briefly reopened at 3am, it was forced to close once more 45 minutes later after the drone flights resumed. As of 9:15AM, Thursday morning, flights to and from the airport remained suspended.

As well as preventing any flights from taking off, the suspension means that numerous inbound flights had to be diverted to other London-area airports including Luton, Heathrow and Stansted, while other flights were forced to land in Paris and Amsterdam. In total, 760 flights containing 110,000 passengers were due to either take off or land from Gatwick over the course of Thursday.

Responding to the incident, the UK’s Civil Aviation Authority said that “it is totally unacceptable to fly drones close to airports and anyone flouting the rules can face severe penalties including imprisonment.” Currently, the law states that flying within 1km(0.6 miles) of an airport is illegal in the UK without explicit permission, and if caught, the pilots of Gatwick’s drones could face up to five years in prison according to the Aviation Minister Baroness Sugg. Although drone users in the US have been required to register their drones since 2015, similar rules for UK owners aren’t due to come into effect until November 2019.