Britain’s second-busiest airport has suspended all flights “due to an air traffic control systems issue”.
The airport said: “We are working with ANS, our air traffic control provider, to rectify this issue as quickly as possible.
“We apologise and advise passengers to check with their airline or on our live flights page for the latest flight information.”
The problem relates to the control tower.
Gatwick has the busiest runway in the world, with a take-off or landing every 80 seconds at peak times.
Arriving aircraft are already being diverted.
A British Airways arrival from Tenerife is currently on the ground at Stansted. Another BA plane, inbound from Ibiza, touched down at Bournemouth.
Many easyJet arrivals were diverted, included services from Montpellier and Seville which landed at Luton, and from Krakow which arrived in Stansted.
The aircraft will remain on the ground in the hope that the Gatwick runway opens swiftly and they can be flown in.
Gatwick is the biggest base for easyJet, which in a normal day operates as many as 400 flights to and from the Sussex airport.
It told passengers: “Due to an earlier IT issue affecting all flights departing and arriving at London Gatwick Airport we expect delays and possible further disruption.
“Although outside of our control, we're very sorry for any inconvenience this may cause.” The airline also warned passengers they would not get any cash compensation: “The disruption is beyond our control and is considered an extraordinary circumstance.”
Even if a swift fix is found to the problem, disruption will continue well into the evening.
The final wave of early evening departures from Gatwick is likely to be wrecked, with many flights cancelled and others severely delayed.
Although passengers do not get cash compensation, they are entitled to meals and, if necessary, accommodation until they can be flown to their destinations.
Shortly before Christmas, Gatwick airport was closed for 33 hours, disrupting the plans of 160,000 passengers, because of unauthorised drone activity.