Firefighters are tackling a blaze at a car garage in Scorton, near Preston.
Six fire engines were called to Woodland Motors on Station Lane at 11.43am.
Several gas cylinders containing acetylene - commonly used for welding - are inside and a 200 metre exclusion zone has been put in place.
The A6 between The New Holly pub and Cabus is closed to traffic in both directions.
Fire crews from Preston, Fulwood, Lancaster, Morecambe and Garstang, are attending.
A spokesman for Lancashire Fire and Rescue Service said: “When firefighters arrived at the scene they saw a building that was approximately 10 metres by five metres on fire and during their initial investigation, it became apparent that there were several cylinders inside the garage, one of which is acetylene.
“Acetylene is commonly found in car garages as it is used for welding but if it is exposed to heat, it can become dangerous with the potential to explode.
“This danger has resulted in firefighters setting up a 200 metre cordon which will be in place until 3.30pm at the earliest this afternoon.
“At the height of the incident, the fire service had six fire engines at the scene plus specialist appliances such as an aerial ladder platform and mobile fire station.
“Whilst much of the fire has now been extinguished, firefighters remain at incident to cool down the cylinders and extinguish any smaller pockets of fire.
“The cause of the fire is still unknown, however, a full fire investigation will begin once it becomes safer for the firefighters to re-enter the garage.”