Migrant workers are being lured to Britain by deception and are forming a new exploited underclass, a BBC investigation has discovered.
Undercover Lithuanian journalist Audrius Lelkaitis discovered a new form of people trafficking, systematic underpayment and exploitation.
Human trafficking experts said the conditions suffered by some migrant workers was "modern-day slavery".
All companies targeted by the investigation deny exploiting workers.
Mr Lelkaitis, working as part of the BBC News investigation, posed as a migrant worker seeking a job in the UK.
He paid hundreds of pounds to agencies in Lithuania and London in return for the promise of a job in Hull which did not exist.
After being offered work with licensed gangmasters Focus Staff Limited in Hull he was paid below the minimum wage two weeks in arrears.
After three weeks, he received £97 for 20 hours' work in his first week, although £50 was deducted for accommodation costs.
He also had money deducted for accommodation without it being shown on his payslip, which is illegal, and was forced to live in overcrowded accommodation.
Mike Dickenson, director of Focus Staff Ltd, denies any wrongdoing
Mike Wilson, chief executive of the Gangmasters' Licensing Authority said the withholding of wages when workers were already in debt sounded "suspiciously like a bonded labour situation which certainly we would not agree to at all".
Mike Dickenson, director of Focus Staff Limited, denied any illegal practices.
"I don't underpay my workers," he said. "Everything I do is legal and above board," he said.
Deputy Chief Constable Grahame Maxwell, programme director of the UK Human Trafficking Centre, said the opening of Europe's borders had brought with it a new kind of people trafficking.
He said: "This quite clearly is labour exploitation. Certain elements are there; there's a deception and there's a movement of people with an expectation of being paid a reasonable and appropriate wage.
"This is a kind of forced or bonded labour. This is modern day slavery."
Aidan McQuade, director of campaign group Anti-Slavery International, said trafficking to exploit labour involved a number of factors.
These included the use of deception, intimidation, the removal of documents, excessive charges for accommodation and transport, the exploitation of someone's irregular immigration status or the fact they are in debt, in order to force them to work in conditions they do not agree to, he said.
He added: "Some of these mechanisms are reported in this BBC News investigation."