Up to 40 firefighters tackled a blaze at a car garage in Scorton, near Lancaster.
Six fire engines were called to Woodland Motors on Station Lane at 11.43am.
Several gas cylinders containing acetylene - commonly used for welding - were inside and a 200 metre exclusion zone put in place.
The A6 between The New Holly pub and Cabus was closed to traffic in both directions but later re-opened.
Fire crews from Lancaster, Morecambe, Garstang, Preston and Fulwood attended.
A spokesman for Lancashire Fire and Rescue Service said the cordon was set up because contents of the cylinders “can be become quite dangerous” when hot with the “potential to explode.”
“The majority of the fire is out but firefighters have to keep squirting water on the cylinders to cool them down,” he said.
Tests will be carried out at 3.30pm to examine the safety of the scene and whether the exclusion zone can be lifted.
The cause of the fire is now being investigated.