Firefighters found a makeshift shelter built near allotments in Harrogate was well alight when they arrived at the scene.
Crews had been responding to reports of a fire spotted near allotments in Bachelor Road at around 6.20pm yesterday.
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A North Yorkshire Fire Service spokesman said firefighters arrived to find a makeshift shelter, most likely built by youths, was well alight.
They used a small jet and knapsack sprayers to put out the flames.
The cause of the fire is believed to be deliberate, the spokesman added.