The International Labour Organization says 2.4 million of them are victims of trafficking, and their labour generates profits of over $30bn.

The ILO says that while the figures may be lower than recent estimates, they reflect reported cases which may rise as societies face the problem.

The report calls for a global alliance to improve laws and raise awareness of what it calls a "hidden" issue.

Global problem

The report, entitled A Global Alliance Against Forced Labour, is the ILO's second major investigation into slavery this century.

The organisation says forced labour is a global problem, in all regions and types of economy.

The largest numbers are in poor Asian countries and Latin America, but there are more than 350,000 cases in the industrialised world. Four-fifths of forced labour is exacted by private agents and most victims are women and children, the ILO says.

The report has uncovered a significant amount of the kinds of forced labour which have been known about for a long time.

An example is bonded labour - where children are forced to do the same jobs as their parents, without hope of release.

Modern slavery is growing in some conflict zones, with the seizure of children as soldiers or sex slaves.

But the report sees the biggest deterioration in the newly globalised economy, in sectors such as the sex industry, agriculture, construction and domestic service.

Local knowledge

The ILO calls for better laws and stronger law enforcement to break "a pattern of impunity" in "privately-imposed forced labour".

HAVE YOUR SAY Slavery is not just a crime... It is a business Jessica, Houghton, US

The reports also urges societies to address the roots of the problem by working with local communities in the poorest countries. The ILO suggests that wealthier countries could tackle the issue by looking at their labour and migration policies.

BBC developing world correspondent David Loyn says there are some positive signs of change.

Increased concern about organised crime has led to a new international protocol against people-trafficking.

Last year, trade unionists from a range of countries met in Cameroon to discuss issues including slavery and abduction, forced domestic labour and the sex trade.

The problem could be resolved in these smaller-scale non-governmental meetings, our correspondent says, because local individuals with business knowledge are more likely to uncover the practice than formal investigators.

But, he adds, it will take a lot to change the culture of forced labour, as it operates best in informal areas outside the view of the normal economy.