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If May can influence other leaders to join her to end this abuse, she will certainly be building a legacy of which she can rightly be proud
As leaders from 53 countries gather in London for the Commonwealth Summit next week, around 10 million children worldwide will be working as slaves - millions of them in the Commonwealth itself.
Children, including those in forced labour and early marriage, number around a quarter of the world's 40.3 million modern slaves, according to the International Labour Organization (ILO).
India, the Commonwealth's most populous country, is well-known for having large numbers of modern slaves, including children in early marriage and bonded labour (watch here). Many of these children are extremely vulnerable and we at World Vision have been working with them and their communities.
But India is not alone in facing such ills as millions of other cases have been documented across African and South Asian countries within the Commonwealth.
And of course the summit's host - we here in the UK - are not immune from modern slavery. In 2015 the UK passed the Modern Slavery Act in a bid to tackle abuse on vulnerable individuals and children. Two years on, Prime Minister Theresa May staked out a leadership role in fighting slavery, launching the Call to Action to End Forced Labour, Modern Slavery and Human Trafficking at the UN General Assembly.
Mrs May's government sees modern slavery as a problem the whole Commonwealth needs to work together on so it is to be welcomed that it is on the agenda for the summit next week.
Clearly if the UK can persuade more Commonwealth leaders attending the summit to join the other 43 signatories to the Call to Action, this would be an important recognition by the international community of the action and resources needed to tackle modern slavery.
However, if the PM really wants to lead the international community in bringing hope to the world's child slaves, the UK government needs to make the links between the Call to Action and another of its major areas of international leadership - humanitarian action.
The ILO recognises that some of the children most vulnerable to slavery forced labour and trafficking, are those living in and fleeing from fragile and crisis-afflicted areas and states. In the wake of conflicts and other emergencies, communities and families lose their livelihoods; schools close and child protection systems break down.
Forced below the breadline and seeing no end to the crisis at home, the impoverished Syrian parents in the in the refugee camps in Jordan or within the host communities in Lebanon may see child labour or early marriage as the best available options for their children. Separated from their families, unaccompanied children are especially vulnerable to traffickers and others who would exploit them.
We know from our work that children who are left behind in conflict and crisis-affected states such as the Democratic Republic of Congo, are also vulnerable to being sucked into joining armed groups, working in mines, and being sexually exploited: all deemed by the international standard to be analogous to slavery.
If the UK and international community want to prevail against the slavers and traffickers, their humanitarian policies and aid programmes should prioritise keeping children in emergencies safe from violence.
Our long experience in these countries tells us that those delivering aid need to be resourced for specific child protection work. Donor countries like the UK should also consider cash transfers and training and employment programmes so that distressed families are not forced to entertain more desperate options for their children.
Mrs May goes into next week's Commonwealth summit aware that this is a test of her leadership on the international stage. If she can influence other leaders to join her to end this abuse of vulnerable children, she will certainly be building a legacy of which she can rightly be proud. If she fails, however, it will be the children who suffer.
Gavin Crowden is the World Vision UK's Head of Policy, Advocacy and Campaigns.