It is illegal for Malaysians to work in Singapore if they do not have valid work passes, said Acting Manpower Minister Tan Chuan-Jin in Parliament on Tuesday.
The ministry conducts random checks and acts on public complaints, he said, but it is not realistic to expect the practice to be completely eradicated.
"In every country, we will always have in our economy certain segments of the economy that perhaps operate rather under the radar scope," said Mr Tan.
He was responding to a question from Mr Baey Yam Keng (Tampines GRC) who wanted to know how many Malaysians cross the Causeway regularly on social visit passes to work in the informal sector here, such as as freelance plumbers, technicians or electricians.
Mr Tan also refuted Workers Party chief Low Thia Khiang's claim that the MOM closes its eyes to such practices, including Malaysians who hold construction work passes but do work not related to the construction sector.
"I do urge Mr Low to provide us with the alleged information about us closing one eye," said Mr Tan.
The problem is not just confined to Malaysians, added Mr Tan.
"There are other people who come to Singapore on a social visit pass and we are aware that sometimes some of them might engage in various types of work whether legal work or otherwise," he said.
Under the Employment of Foreign Manpower Act, employers who hire foreigners without valid work passes could be fined up to $20,000 or jailed up to two years, he said. The illegal workers may also be banned from entering Singapore for up to two years.