The runway at Gatwick has been closed to all flights because of a 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.
An airport spokesman confirmed that by 10.55am about six incoming flights had been diverted.
He said the "priority" will be for incoming flights to land as soon as safety checks have taken place, adding that the runway will reopen "shortly".
Flight tracking data appears to show at least three incoming flights circling in the skies above southern England, including British Airways flight BA2957 from Glasgow which was due to land at 11.05am.