By Maddie Drury

LONDON, June 11 (Thomson Reuters Foundation) — Britain's prime minister will on Tuesday urge global leaders to do more to tackle modern slavery, saying they have a "moral duty" to combat a crime estimated to affect more than 40 million people worldwide.

Theresa May will pledge £10 million ($13 million) to stop children being exploited in agriculture in Africa and announce plans for a new international envoy to coordinate Britain's anti-slavery efforts, her office said in a statement.

"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," May will tell leaders, the statement said.

"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," she will say at an international conference in Geneva.

The event marks the centenary of the United Nations International Labour Organisation, which estimates that there are 40.3 million people living in slavery worldwide.

Britain is seen as a global leader in its efforts to end slavery, having passed the landmark Modern Slavery Act in 2015 to jail traffickers for life and compel large businesses to address the threat of forced labour.

But the law is under review amid concerns about its effectiveness in driving prosecutions, supporting victims and ensuring businesses stop using forced labour.

May, who will stand down as prime minister when her party chooses a new leader, will say it is crucial to "match words with actions" if the world was to meet a UN goal of ending modern slavery by 2030.

She will also announce the creation of a new central registry of modern slavery transparency statements by large businesses to help consumers make more informed choices.

Britain's anti-slavery law requires companies whose turnover exceeds £36 million to file an annual statement detailing the actions they have taken to stop slavery in their operations.

"We hope the new leader can address the areas where Ms May fell short, particularly in building an effective system of protection for victims," Jakub Sobik, spokesman for the charity Anti-Slavery International, told the Thomson Reuters Foundation.

($1 = 0.7861 pounds)

(Reporting by Maddie Drury. Editing by Claire Cozens. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers that covers humanitarian news, women's and LGBT+ rights, human trafficking, property rights, and climate change. Visit http://news.trust.org)