The government is leaving migrant workers at the mercy of gangmasters, a new report has said.

TUC research reveals that a lack of legal protection for thousands of immigrants is "giving the green light to exploitation".

The trade union body calls on ministers to sign the UN Convention on the Rights of Migrant Workers and their Families, which would offer safer guarantees for migrants' protection from employers.

The report, entitled "Overworked, underpaid, and over here", argues that workers are being bullied into long hours and low pay because they must often remain beneath the radar of authorities.

Official figures show that the working population born outside the UK has increased from seven per cent to nine per cent between 1995 and last year.

TUC campaigners say this is "clearly an underestimate as it will not include the many people who work here illegally".

Migrant workers are employed in almost every corner of the UK, with particular concentrations in London and the South East, East Anglia and Scotland; work in health care, education, cleaning, food manufacture and agriculture, hospitality, IT and construction; and often include foreign students, the authors say.

While workers are often exploited because of their ignorance of the law and their employment rights, the report argues, the situation is even worse for those working without legal permission to do so.

It is likely, said the TUC, that these workers have no rights at all, with tribunals declining to hear complaints from workers without legal contracts.

"Popular perceptions that the UK is home to thousands of migrant workers, who plan to stay long term, and take the jobs of UK workers are simply wrong," TUC general secretary Brendan Barber said.

"The reality is that most migrant workers only stay for short periods, and their precarious legal status means many end up working incredibly long hours for not much pay, in jobs that UK workers wouldn't want to do.

"The challenge for unions is to find ways of recruiting migrant workers, offering them support and guidance so they become less exploitable and more aware of their rights."