London (CNN) More than 40 million people were estimated to be victims of modern slavery in 2016 -- and one in four of those were children.

Those are the findings of a new report produced by the International Labor Organization (ILO), a U.N. agency focusing on labor rights, and the Walk Free Foundation, an international NGO working to end modern slavery.

The report estimates that last year, 25 million people were in forced labor -- made to work under threat or coercion -- and 15 million people were in forced marriage.

It's impossible to know exactly how many people are living in modern slavery, and different studies have produced different estimates. One reason is that modern slavery is a hidden crime that's difficult to identify. Another is that different studies use different definitions of slavery, with some including forced marriage, for example, and others not.

In the past, two of the most widely quoted figures have been those produced separately by the ILO and the Walk Free Foundation. But this report, called The 2017 Global Estimates of Modern Slavery , marks the first time the ILO and the Walk Free Foundation have collaborated to produce worldwide slavery figures.

We know that if there are 40 million people in modern slavery, only tens of thousands of victims are being helped ... Fiona David, Walk Free Foundation

Both organizations describe the report as giving the most reliable slavery figures to date, and the researchers say the numbers show that much work is needed to combat the problem.

"We know that if there are 40 million people in modern slavery, only tens of thousands of victims are being helped, assisted and supported, whether through the criminal justice system or through victim support systems," Fiona David, Walk Free Foundation's executive director of global research, told CNN. "It's a massive gap that we have to close."

Women and children

According to the report, women and girls accounted for 71 percent of slavery victims, including 99 percent of those in the commercial sex industry and 84 percent of victims of forced marriages.

Children made up around 37 percent of those forced to marry, as well as 18 percent of forced labor victims and 21 percent of victims of sexual exploitation.

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The report defines modern slavery as situations of exploitation that a person can't refuse or leave because of threats, violence, coercion or deception -- which includes forced labor, debt bondage, forced marriage and human trafficking.

It notes that the average length of time victims were in forced labor ranged from a few days in some forms of state-imposed forced labor, to nearly two years for forced sexual exploitation.

An estimated 4.1 million people are victims of forced labor imposed by state authorities. They include people made by their state authorities to participate in agriculture or construction work to boost economic development, young military conscripts forced to perform non-military work, and prisoners forced to work without consent at private prisons.

How they got their numbers

The report combined data collected from 2012 to 2016 and drew on a variety of sources, chiefly, national surveys involving interviews with more than 71,000 respondents across 48 countries.

It also used data from the International Organization for Migration -- an intergovernmental organization related to the U.N.

The report found that modern slavery was most prevalent in Africa (7.6 victims per 1,000 people), followed by Asia and the Pacific (6.1 per 1,000).

Forced labor was most prevalent in the Asia and the Pacific region, (4 victims per 1,000 people), and forced marriage was most prevalent in Africa (4.8 per 1,000), followed by Asia and the Pacific (2 per 1,000).

However, the report warns that these regional figures should be interpreted with caution, because of a lack of available data in some regions, notably the Arab States and the Americas.

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The report identified forced labor in all kinds of industries. In the cases where the type of work was known, 24 percent of adults were domestic workers, 18 percent were in construction, 15 percent in manufacturing, and 11 percent in agriculture and fishing.

There is a need for labor and social rights in the informal economy, and a need for collective bargaining and workers' organizations Michaelle De Cock, ILO

An estimated 3.8 million adults were victims of forced sexual exploitation and 1 million children were victims of sexual exploitation.

Michaelle De Cock, senior statistician with the ILO, told CNN that the research on forced labor highlighted a lack of social protection systems. "There is a need for labor and social rights in the informal economy, and a need for collective bargaining and workers' organizations," she added.

Child labor

The ILO simultaneously released another report, produced without the Walk Free Foundation, called The 2017 Global Estimates of Child Labor . It found that there are 152 million children around the world engaged in child labor, and 73 million of them are in work that "directly endangers their health, safety and moral development."

According to the report, 90 percent of all children in child labor are in the Africa and the Asia and the Pacific regions. In Africa, 20 percent of the continent's children are in child labor, while in the Asia and the Pacific region, 7 percent are in child labor.

More than two thirds of all children in child labor work on family farms or for family businesses. The report noted that "understanding and addressing family reliance on children's labor will therefore be critical to broader progress towards ending child labor."

It also found that 71 percent -- 108 million -- of all those in child labor work in agriculture -- generally in subsistence and commercial farming and livestock herding.

The report uses data from 105 national household surveys, covering more than 70% of the world population of children aged 5 to 17 years.

Included in the figures are almost 38 million children aged 15-17. Although 15 to 17-year-olds are above the minimum working age of 15, the ILO considers them to be in child labor if their work may be physically or psychologically injurious to their well-being.

The two reports are intended to help towards achieving target 8.7 of the U.N. Sustainable Development Goals (SDGs). The SDGs are 17 broad goals to be achieved by 2030 and target 8.7 calls for effective measures to end forced labor, modern slavery, and human trafficking, as well as child labor in all its forms.

"The SDGs are very much the motivator for this research," said David. "It's critical that if you want to end child labor and modern slavery then you have to have some measurement framework, so we're trying to get a handle on how big the problems are so everyone has the information they need to develop responses."