Military personnel were rushed to the private company in the commercial quarter of Grandpass to search for those crushed inside.
Police said the cause of the accident was still unclear but investigations were underway into whether unauthorised extensions had been made to the single-storey building.
The government has earmarked for demolition around 10,000 illegally constructed homes and commercial buildings in Colombo after a seven-storey wedding hall collapsed in May, killing one and wounding 23.
High property prices and the scarcity of buildable land has seen haphazard and illegal construction spike in the city of 750,000 people.