Firefighters were called to the scene on Towngate shortly before 3pm on Saturday.
The scale of the incident meant eight fire engines were dispatched to tackle the flames.
A spokesman for Lancashire Fire and Rescue said: "The fire involved a single storey industrial farm building approximately 50m by 40m which contained cylinders.
"Firefighters have sectorised the incident and are tackling the fire using ground monitors.
"The aerial support unit is also currently in attendance assisting with firefighting operations."
More to follow