The UN warned on Tuesday that it had found evidence of human trafficking in 118 countries and the victims, mostly women, were of 136 different nationalities.
A new UN Office on Drugs and Crime (UNODC) report said that the millions of victims could be found working in the world's restaurants, fisheries, brothels, farms and homes.
It said that trafficking for sexual exploitation accounted for 58 per cent of all cases, while the share of forced labour cases had doubled over the past four years to 36 per cent.
"This global crime generates billions of dollars in profits for the traffickers," said UNODC executive director Yuri Fedotov.
The International Labour Organisation estimates that 20.9 million people are victims of forced labour globally, including victims of human trafficking for sexual exploitation.
"While it is not known how many of these victims were trafficked, the estimate implies that there are millions of trafficking victims in the world," said Mr Fedotov.
Women and girls accounted for three-quarters of all trafficking victims. And the proportion of child victims rose from 20 per cent between 2003-6 to 27 per cent between 2007-10.
Other forms of trafficking remain relatively rare.
Trafficking for organs, though just 0.2 per cent of cases in 2010, was reported by 16 countries.
Trafficking for other purposes including begging, forced marriage, illegal adoption, participating in armed combat and crime accounted for 6 per cent.
The UNODC said progress had been made in fighting trafficking, with 134 countries and territories passing laws criminalising it, but there had been few convictions.
Of the 132 countries covered in the report, 16 per cent did not record a single conviction for human trafficking between 2007 and 2010.
Mr Fedotov said: "Trafficking requires a forceful response founded on assistance and protection for victims, rigorous enforcement by the justice system, a sound migration policy and firm regulation of labour markets."
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