Two fire engines and crews, from Chorley and Bamber Bridge, had been called to the blaze, at a double garage in Hunt's Field, at 3.43am.
They were forced to use a water jet to suppress the fire until gas technicians arrived to shut off the supply to the outside gas pipe, a spokesman for Lancashire Fire and Rescue Service (LFRS) said.
He described the rupture as a 'complication'.
Nobody was hurt, and the cause of the fire is now under investigation.