Theresa May is announcing a new transparency registry at a UN summit in Switzerland that will include listing companies using slaves - so shoppers are aware.
The plans also include £10m of government funding to prevent children in Africa being exploited by the farming industry.
David Westlake, CEO of anti slavery organisation IJM told Premier: "Well, I want to start by saying this is an area where Theresa May has actually made a stand and led the world really in terms of naming modern slavery as an issue.
"In your News Hour, today, well, while this 60 minutes goes on, 120 children will be sold into slavery. And that's sold into sweatshops and factories and brothels, and of course children should be in families and playgrounds and schools.
"This is an area where she has had some moral courage and led the world."
In 2014, the Home Office estimated there was between 10,000 and 13,000 potential victims in the UK - just 2,340 of those were officially reported and recorded.
Mrs May, who stepped down as leader of the governing Conservative Party earlier this month, is also due to call on world leaders to face up to their "moral duty" and take action to halt modern slavery.
Speaking at the United Nations' International Labour Organisation centenary conference in Geneva, Mrs May will urge business and political representatives to do more to protect the millions of people being held and forced to work against their will.
She is expected to say: "No leader worthy of the name can look the other way while men, women and children are held against their will, forced to work for a pittance or no pay at all, routinely beaten, raped and tortured.
"So, those of us who can speak out, who have a platform from which to be heard, have a duty, a moral duty, to raise our voices on their behalf."
Mr Westlake said it is important for consumers to ask questions when making purchases, such as: "Who made the clothes I wear? Who picked and processed the food I eat? Was slavery involved in that?"
Mrs May will argue it is "more important than ever" to "accelerate" the fight against modern slavery and "do all that we can to meet the UN's goal of ending this abhorrent crime by 2030".
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