Agriculture uses many migrant workers

The TUC study said many migrant workers in the UK receive "shocking" treatment from their employers.

It reported cases of employers holding on to the passports of their staff and intimidating workers.

Migrant workers should be granted the same right to leave unsuitable jobs as UK workers, the TUC said.

Physical assault

People with every legal right to hold a job can also be so badly exploited that they must count as forced labour Brendan Barber, TUC general secretary

The report, "Forced labour and migration to the UK", published ahead of the first anniversary on Saturday of the Morecambe Bay cockling tragedy, cited the experience of illegal and legal migrant workers.

In one case a group of Eastern Europeans was given false passports and threatened with physical assault if they tried to leave the factory where they worked.

Others cases highlighted included workers being forced to work off the cost of their passage to the UK, having their passports held by their employer or being threatened with deportation if they complained about poor conditions.

The report also highlighted the plight of some legal migrant workers.

It said legal migrant workers are regularly subjected to long hours, pay below minimum wage levels and dangerous working conditions.

"What this report shows is just how much people with every legal right to hold a job can also be so badly exploited that they must count as forced labour," Brendan Barber, TUC general secretary, said.

Work rights

The International Labour Organisation identifies forced labour as occurring whenever workers are subjected to physical threat, restriction of movement or withholding of wages.

The report called for employees working on a work permit in the UK should be able to change job, and that employers found guilty of abuses should lose their right to apply for work permits for their staff.

At present, if a migrant worker leaves their contracted employment they risk losing their right to remain in the UK.

In November, the Home Office said up to 600,000 migrant workers were needed to fill job vacancies in the UK.