Four fire engines and around 20 firefighters tackled a fire in the basement of a five-storey office block on Hay's Lane near London Bridge on Monday morning.
The London Fire Brigade says that an electrical junction box in the basement was damaged by fire.
Approximately 50 people were evacuated from the building before the firefighters arrived.
The brigade was called at 8.58am on Monday and the fire was under control just after 10am.
Crews from Dowgate, Southwark, Old Kent Road and Whitechapel stations were at the scene.
The cause of the fire is under investigation.