Some 12.3 million people are enslaved worldwide, according to a major report.
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.
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
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
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.