Three fire engines were called to a fire on Thursday night in the beer garden of the Walrus Social bar on Lower Marsh.
Garden furniture and structures were damaged in the fire which the bar's owner believes was started by burglars targeting the adjacent nail bar.
Two gas cylinders were found in the premises close to Waterloo Station and a temporary hazard zone was set up around the affected area as a precaution as some cylinders can explode when exposed to heat.
The brigade was called at 8.38pm and the incident was under control just after 10pm.
Crews from Lambeth, Westminster and Southwark fire stations were at the scene.
The cause of the fire is under investigation.