Subsidies of around $3.9 billion a year being paid out to US cotton farmers are at the heart of a deep crisis in global markets according to Oxfam, the charity that campaigns for better global trade deals for poorer countries.

Speaking out in support of Brazil, which has filed a challenge against the subsidies at the World Trade Organisation (WTO), the UK-based development charity said the financial support for 25,000 US cotton farmers is pushing millions of sub-Saharan Africans into poverty.

It also accuses the subsidies of encouraging overproduction and export dumping, and driving down world prices.

In its report, 'Cultivating Poverty - The Impact of US Cotton Subsidies on Africa,' Oxfam said  total US government support was more than three times the total of all US foreign aid to Africa's 500 million people.

In its WTO complaint, Brazil said the support programme enabled US cotton giants to boost their share of the global market even though their production costs were the world's highest.

Oxfam added that the subsidies allow US cotton-producing firms to undercut more efficient farmers in poor states. More than 10 million people in sub-Saharan Africa depend on cotton production it said.