AIRLIVE
Radars show the plane currently at 8,000ft above land and flying over Brighton in Sussex.
Airline website Airlive reported the plane gave a Squawk 7700 - a general emergency.
The plane is understood to be heading to Gatwick Airport.
The Airbus A319 was due to land at Luton at 11.50am. Website FlightRadar24 a medical emergency on board the jet is believed to be the reason for the diversion.