An explosion at a former fast food restaurant in Shrewsbury town centre happened when workmen were removing piping, it is believed.
The blast happened at the old Burger King site on Pride Hill, which is being refurbished, on Monday morning.
Shropshire Fire and Rescue Service say they had the situation under control within an hour.
The nearby post office was evacuated following the small explosion but nobody was hurt in the incident.
The fire service were called at about 0930 GMT and they said the damage from the fire "wasn't substantial".