Once praised for its success in containing the virus, Singapore is now facing a surge of infections linked to industrial worksites and tightly packed worker dormitories.
On Thursday, 728 new cases were reported, bringing the total number of cases to 4,427.
Almost 90% of these infections have been linked to these workers.
The number is expected to rise as more workers get tested in the coming days to help identify the scale of the problem.
Around 300,000 low-wage workers, mostly from South Asia, work in Singapore in construction and maintenance.
Most of them live together in huge dormitory complexes on the outskirts of the city they've helped build.