Firefighters tackled a heath fire near Ferndown Middle School on Friday afternoon.
Crews from Ferndown and Poole were called at around 3.05pm to reports that a small area of land ablaze.
They discovered a fire measuring approximately 50 square metres, and quickly began to extinguish the flames using beaters.
Investigators have now confirmed that they believe the fire was started deliberately.
Crews left the scene at 5.37pm.
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