Some 21,000 people in the Netherlands work in conditions of virtual slavery, with long days for little money, according to a report by the FairWork foundation to be published on Thursday.
The figure is based on research by the International Labour Organisation, which says 880,000 people in Europe are seriously exploited at work.
While migrants are most vulnerable, Dutch nationals can also be affected, the NRC quotes the foundation as saying. Hidden slavery is most likely in the hospitality industry, farming, construction and domestic services.
Earlier this year, an asparagus grower was jailed for three years for human trafficking and exploiting workers on her farm in the Brabant village of Someren.
The court said the woman had forced people to work seven days a week, 10 to 14 hours a day and paid them less than the minimum wage. They were unable to leave the farm because J withheld their earnings until the end of the season.