More than one third of all victims of forced marriage were children at the time of the marriage, and nearly all child victims were girls.
An estimated 15.4 million were also living in forced marriages in 2016 - of this total, 6.5 million were Wednesday in the period 2012-2016.
International Organization for Migration (IOM) and the group Walk Free will be releasing the 2017 estimate of modern slavery and child labor around the globe. Nearly one in five child labourers work in the services sector (17.1 per cent) while 11.9 per cent of child labourers work in industry.
Women represented 99 percent of the victims of forced labour in the commercial sex industry and 84 percent of forced marriages, according to the study.
Women and girls are disproportionately affected by modern slavery, 71 percent of the overall total, a study says.
A landmark new estimate of the number of people around the world trapped in modern slavery could galvanise a global drive to end the worldwide crime by 2030, experts said on Tuesday.
"The fact as a society we still have 40 million people in modern slavery on any given day shames us all".
The toleration of forced marriage, particularly that of children, represents such a fundamental denial of the rights of millions of girls that it provides a fertile ground for the evolution of yet more egregious abuses such as the misogynistic depredations of Boko Haram and Islamic State. About 37 percent (or 5.7 million) of those who were forced to marry were children.
The estimate compared with a 2016 Walk Free finding that 45.8 million people were slaves and an ILO figure of 21 million in forced labor, but both the ILO and Walk Free cautioned the latest number can not be compared with earlier figures to show progress or failure in anti-slavery efforts.
The treatment of millions of girls who were forced to marry against their will finally be recognised for what it is - slavery, hidden under the guise of marriage.
The ILO also released a separate report showing 152 million children were victims of child labor, which amounted to nearly one in every 10 children worldwide with almost half of them engaged in hazardous work.
Half of forced laborers were victims of debt bondage, who were made to work to repay a debt or other obligation, and almost four million adults and one million children were victims of forced sexual exploitation.
The term "modern slavery" has not been defined by any global instrument.
Modern slavery was most prevalent in Africa (7.6 per 1,000 people), followed by Asia and the Pacific (6.1 per 1,000) then Europe and Central Asia (3.9 per 1,000).
The report has triggered calls for stronger labor rights, improved governance of migrants and action to address root causes of debt bondage, and better victim identification.
Previously the groups had used different data, definitions and methodologies, said Houtan Homayounpour, a specialist on forced labour at the ILO, the United Nations' labour agency.