An investigation has been launched after wooden pallets were deliberately set alight outside a shop in Preston City Centre, say fire services.
Two fire crews from Preston were called out to the back of B&M stores on Tithebarn Street at around 7.45pm on September 18.
Firefighters arrived to find a well established fire with smoke pouring into the building.
A spokesman for the fire service said: "When we got there there was quite a lot of smoke and flames and the fire was right against the B&M store wall.
"There was a very real risk that the fire could have spread to the building and there could have been a lot of shops involved.
"Thankfully a passerby notified the fire service in time.
"We used a hose reel jet to extinguish the fire and two large fans to clear the buildings of smoke.
"Arson is a very serious crime and it takes up unnecessary time and resources for firefighters who may be needed at other incidents.
"We're asking anyone with information to please contact police."
Old Vicarage was closed while crews spent around one -and-a -half hours dealing with the fire.
Nobody was injured during the incident.