Agriculture and food processing are two of the most affected industries
The number of foreign workers being exploited by employment agents is far higher than was previously thought, the Gangmasters Licensing Authority says.
The GLA, set up in 2005 to protect low-paid workers, says thousands of immigrants have worked in appalling conditions for below the minimum wage.
The authority says it has revoked 25 employment agents' licences already this year - twice as many as last year.
In response, it has launched a major operation to target rogue employers.
Operation Ajax will include up to 30 raids during the next 18 months, across the UK.
The GLA currently licenses 1,200 gangmasters, but since its creation it has revoked the licenses of 57.
The problem of exploitation by unscrupulous employment agents is particularly acute in the food processing and agricultural industries, says the GLA.
It reports people being forced to work in unsafe conditions, often for wages well below the legal minimum, and workers having their families back at home threatened.
The authority's chairman, Paul Whitehouse, said: "Operation Ajax is a long-term commitment to enforce worker rights and stamp out exploitation wherever and whenever we find it.
"Based on our intelligence we have put together the operation to root out the rogues and catch the crooks."
TUC deputy general secretary Frances O'Grady welcomed the project's launch, but said not all vulnerable workers were protected by the GLA.
"We believe its targeted and effective approach to stamping out rogue employers should be extended to other sectors, where we know extreme abuse remains rife."
The GLA was set up in 2005 to curb abuse of workers in agriculture, shellfish gathering and food processing, following the deaths of 23 Chinese cockle pickers in Morecambe Bay, Lancashire, a year earlier.
The cocklers' gangmaster, Lin Liang Ren, who is originally from China, was later convicted of the manslaughter of 21 of them after they were caught in high tides while scouring for cockles.
The GLA has launched investigations into abuses, including workers being charged for transport, living in substandard and overpriced accommodation and being paid below the national minimum wage.
These cover about 500,000 workers in industries such as fruit picking and food processing.