Essex firefighters were called to the hangar, which is used to livery planes, near the terminal shortly after 10.40am yesterday and believe the fire was caused “accidentally”.
Early reports from an eyewitness suggested they had heard an “explosion”, while others denied hearing a bang.
There was an explosion at an hanger on the north side of London Southend Airport as Essex firefighters were called to the scene near the terminal.
An Essex Fire and Rescue Service spokesperson said: “When firefighters arrived at the scene they reported the large hangar, which is used to livery aeroplanes, was on fire.
“Six fire engines were requested to help tackle the fire. Firefighters are using a aerial ladder platform to pour water on the large building from above.
The latest update from the fire service, at 12.40pm, reports that the flames are almost extinguished.
The spokesman added: “A full investigation will be carried out to find the cause of the fire, however at this stage we believe the fire started accidentally.”