The runway at Gatwick has been closed to all flights because of a suspected oil spillage, a spokesman for the airport said.
No flights have been able to land or take off at the West Sussex airport since around 10.15am.
At least 18 incoming flights have been diverted since the runway was shut and many departing services are delayed.
An airport spokesman said the "priority" will be for incoming flights to land as soon as safety checks have taken place.
He added that the runway will reopen "shortly".