Slavery is not a crime in almost half the countries in the world, a study of global laws says, urging states to close legal loopholes that allow abusers to escape punishment.

Many countries lack laws that directly criminalise and punish exerting ownership or control over another person, according to the Antislavery in Domestic Legislation database launched at the United Nations headquarters in New York.

"Slavery is far from being illegal everywhere and we hope our research will move the conversation beyond this popular myth," said Katarina Schwarz, a researcher at the University of Nottingham's Rights Lab, which led work on the slavery database.

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"It will surprise many people to learn that in all of these countries there are no criminal laws in place to prosecute, convict and punish people for subjecting people to the most extreme forms of exploitation."

More than 40 million people are held in modern slavery, which includes forced labour and forced marriage, according to estimates by the International Labour Organisation and the anti-slavery group the Walk Free Foundation.

But although historical laws that once allowed slavery have been scrapped worldwide, researchers for the database said that many of the 193 UN member states have not gone on to explicitly criminalise slavery and other exploitation.

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There is no criminal law against slavery in 94 countries - almost half of UN states - said researchers at Rights Lab, which reviewed the study's findings with the Castan Centre for Human Rights Law at Monash University in Australia.

It concluded that almost two-thirds of countries apparently failed to criminalise any of the main four practices associated with slavery - serfdom, debt bondage, forced marriage and child trafficking - except in the context of human trafficking.

If you or someone you know is impacted by sexual assault, family or domestic violence, call 1800RESPECT on 1800 737 732 or visit 1800RESPECT.org.au. In an emergency, call 000.