On the eve of the first U.N. global forum on human trafficking in Vienna, Oscar-winning actress Emma Thompson said it was time end what the U.N. says is a multi-billion dollar market.

"It is increasing big, big, big time," Thompson, chairwoman of human rights group the Helen Bamber Foundation. "It's the third largest shadow economy after drugs and arms."

The U.N. says some 2.5 million people are trapped in forced labour, including sexual exploitation, in forced marriages, or are pushed to provide body parts for black market organ trade.

About 1,200 government and NGO representatives, legislators, business leaders and trafficking victims from 116 countries will debate over the next three days how to tackle the problem.

They will look at how to track and block Internet payments for trafficking transactions, use technology to pinpoint frequently used routes, establish support networks for victims and create a code of conduct to curb sex tourism.

The surge in human trafficking coincides with a revolution in affordable transport and instant communication around the world, said Antonio Maria Costa, executive director of the United Nations Office on Drugs and Crime (UNODC).

"All of this has facilitated things like trade and services, (yet it has) also facilitated the trafficking of human beings," Costa told a news conference.

A U.N. anti-trafficking protocol took force in 2005, establishing a framework for a crackdown, and now governments must create laws and prosecute perpetrators, he said.

"Governments have not done much. But the international agreement puts a burden on the countries," he said.

The organisers will push for a universal ratification of the U.N. protocol, which has been signed by more than 110 states. There is also a pressing need for better data, as most has come so far from sources like media reports, said Costa.

Most trafficking victims come from nations in the Commonwealth, central and southeastern Europe, West Africa and Southeast Asia, according to a recent UNODC report.

Western Europe, North America and Western Asia top the list of destinations for victims of human trafficking.

Adult woman and girls are most at risk of becoming victims and sexual exploitation is the most prevalent abuse, it said.