The authorities are investigating an errant employer after a surprise inspection of its unlicensed dormitory on Sunday night showed it was housing its foreign workers in cramped, hazardous conditions.

They were found to be living in containers without proper cooking or sanitation facilities.

Worker advocacy group Migrant Workers' Centre (MWC) made an unannounced visit on Sunday night to the makeshift dormitory in the east of Singapore, after receiving a distress call on its helpline from a worker living there.

Accompanied by the police and officers from the Ministry of Manpower (MOM), the MWC conducted a check on the dormitory and found the living conditions did not comply with regulations.

MWC chairman Yeo Guat Kwang said the conditions were not sanitary and did not provide the minimum personal space, noting: "No proper amenities for cooking and storage were provided, despite the fact that cooking took place on site. There were possible fire risks with overloading of electrical points exacerbated by the overcrowding; washing and toilet facilities were also not adequately provided."

The dormitory, located in a large warehousing facility, comprised container units with 10 to 12 workers living in each. More than eight containers were visible, some stacked one on top of another.

The workers, of various nationalities and working mostly in construction, slept on bunk beds in conditions described by the MWC on its Facebook page, as "cramped and dirty". It found only four showers and a single toilet on the site shared by as many as 100 workers at times.

Mr Yeo said at the time of MWC's visit, the housing facility, where the containers were located, were padlocked with the workers inside. This would have meant they had no way of getting out in an emergency.

He said the worker who had made the distress call had expressed other concerns, including restrictions on their movements by their employer, and also salary issues. He declined to elaborate further "for fear of prejudicing investigations or the safety and well-being of the workers". MOM said it visited the premises on Sunday night following a tip-off from MWC, and is now investigating the case. While these are going on, the employer usually must house the workers in other, approved housing.

From January to last month, MOM conducted more than 2,600 housing inspections, and took enforcement action against more than 2,100 employers.

Additional reporting by Linette Lai

Clarification: An earlier version of the story said at the time of MWC's visit, the container units were padlocked with the workers inside. NTUC has clarified that it was the housing facility, where the containers were located, that was padlocked with the workers inside.