Three cars were set on fire and another had its windows smashed in a deliberate attack at HMP Preston, say fire services.
Crews from Preston were called out following reports of the fires in the prison car park on Ribbleton Lane at around 11.30pm on Tuesday March 20.
Firefighters used a hose reel to extinguish the cars - one of which crews say was severely damaged in the incident.
Acting Crew Manager Adam Darrell said: "Several cars were targeted and we believe accelerant.
"One firefighter wearing breathing apparatus used a hose reel to extinguish the fires.
"Three cars were damaged by fire and another had its windows smashed.
"The three fires were started as separate incidents. One car was severely damaged by fire and the other two suffered paintwork damage.
"This was a potentially serious incident as the fire could easily have spread to other vehicles.
"It was good that we stopped it from spreading when we did."
A Prison Service spokesperson said: "There was a fire in the car park. The incident was swiftly dealt with by the fire service, and has been referred to the police for investigation.
“There were no injuries to staff or prisoners, and no disruption to the prison regime.”
A joint police and fire service investigation into the incident has been launched.
Crews spent around one-and-a-half hours at the scene.
Nobody was injured.