Isle of Wight fire crews, from Cowes and Ryde, are among those at Pound Lane near Calbourne.
UPDATE and PHOTOS: Warning after Isle of Wight fires in Brighstone and Calbourne
What was initially feared to be a barn fire was is actually a horsebox on fire in a wooded area, said Hampshire and Isle of Wight Fire and Rescue Service.
Four fire engines and a water carrier are currently on the scene.
The fire has also affected a nearby wooded area and field.
Firefighers are using hose reels and beaters to tackle the blaze, after the were called out at about 4.30pm.
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