A fire at Edwards Buildbase in Wisbech is believed to have been started by an incinerator.
Three fire crews went to the scene on Elm Road after the fire was reported at 12.33pm and found a large quantity of insulation well alight.
The fire was in the open and located next to a building, measuring 25m by 10m, which was heavily smoke logged.
Firefighters wearing breathing apparatus are using hose reels and jets to extinguish the fire. It was extinguished by 1.40pm and crews returned to their stations by 2.35pm.
Matt Murdoch, Station Commander, said: “A large amount of sheet insulation was well alight when we arrived and was in very close proximity to an adjacent warehouse.
“Without the quick interventions from the Wisbech crews who were first on the scene, the fire would have easily spread into the building and possibly even the nearby motorcycle shop.”