Four fire engines were called to Bermondsey's Grange Road on Wednesday afternoon to tackle a blaze in the basement of The Fort.
The brigade was called just after 2.35pm and the fire was brought under control two hours later.
Five people left the building before firefighters arrived on the scene.
Earlier this year Southwark Council refused planning permission for a new block of six flats on the site of the former pub which was most recently used as a gay club.
Planing officers ruled that the proposed development "would represent an oppressive form of development that would result in an increased sense of enclosure detrimental to the residential amenities of the adjoining occupiers".
The cause of the fire is under investigation.
• Whilst tackling the fire, members of red watch from Southwark Fire Station heard the news that the Mayor of London intends to press ahead with the closure of their station and nine others in London.