Millions of people face starvation because of soaring global food prices, putting overwhelming strain on humanitarian agencies, charities warned last night ahead of a world hunger summit in Downing Street.

Severe droughts in the United States and Russia and monsoons in India have wiped out crops, leading to a doubling of the cost of wheat in two months and record prices for corn and soya.

David Cameron will be told that developing countries that rely heavily on food imports will be hardest hit by the international price spike. The No 10 summit will set targets for reducing malnutrition among youngsters by the time of the 2016 Olympics in Brazil.

But charities say they face an immediate crisis that threatens to stretch their resources to breaking point, with severe food shortages facing more than 50m people in Sudan, South Sudan, Somalia, the Democratic Republic of Congo, Afghanistan, Kenya, Zimbabwe and Yemen.

They are reporting that starving children in sub-Saharan Africa are eating roots to survive, while in Yemen families are being forced to marry off young daughters to reduce the number of mouths to feed.

Oxfam said millions of people who are currently "just getting by" will start going hungry unless urgent action is taken. Max Lawson, Oxfam's Head of Policy, said: "Short-term action alone is not enough. The scale of current and recent crises shows there is something fundamentally wrong with the way we produce and distribute food – we cannot keep relying on humanitarian agencies to pick up the pieces."

Save the Children warned that a sharp price rise would push millions more children into hunger as many of the world's poorest families already spend two-thirds of their income on basic staples.

Brendan Cox, its director of policy, urged Mr Cameron to champion the issue of child malnutrition when Britain assumes the presidency of the G8 next year. He said: "This summit has to be the starting gun rather than the finishing line."