Unions were today urged to work with the government in protecting vulnerable workers.
Business secretary John Hutton announced new measures to detect and guard against exploitation of the most vulnerable workers.
Employers caught underpaying workers will now risk unlimited fines, while the number of inspectors is set to double to ensure more employers are caught. The Employment Agency Standards Inspectorate will also receive greater powers.
Speaking at the TUC Congress in Brighton, Mr Hutton trailed new legislation that will prevent agency bosses charging workers for accommodation and transport.
Unions had warned such practices left employees with little or no money, undermining the point of the minimum wage.
But Mr Hutton defended employment agencies, despite the criticisms levied against them by unions, for providing important flexibility in the labour market.
He said: "Vulnerable workers can be exploited by employers prepared to flout the law no matter the nature of their employment or which sector they work in.
"And we know it is completely wrong to suggest that agency workers are automatically victims of abuse and exploitation.
"So we must route out the rogues, whichever sector they are in, as we also act to protect jobs and flexibility in our labour market that offers choice to millions of workers."
Coming into force next month, Mr Hutton also reminded delegates the government has moved to stop employers forcing employees to take Bank Holidays out of their holiday entitlement, with the guarantee of four extra days paid holiday.
Promising the government would take action where needed, he said trade unions must also play an important role in reporting abuse.
Unions must "help shine a light into the dark corners of the labour market and rid Britain of practices that have no place in a modern economy."
The business secretary announced the union modernisation fund, attacked this week by the Conservatives, would receive another £3 million to help unions protect vulnerable workers.