"We will have a problem in coming months," said Josette Sheeran, the head of the UN’s World Food Programme (WFP). "We will have a significant gap if commodity prices remain this high, and we will need an extra half billion dollars just to meet existing assessed needs."

With voluntary contributions from the world’s wealthy nations, the WFP feeds 73 million people in 78 countries, less than a 10th of the total number of the world’s undernourished. Its agreed budget for 2008 was US$2.9bn (£1.5bn). But with annual food price increases around the world of up to 40% and dramatic hikes in fuel costs, that budget is no longer enough even to maintain current food deliveries.

The shortfall is all the more worrying as it comes at a time when populations, many in urban areas, who had thought themselves secure in their food supply are now unable to afford basic foodstuffs. Afghanistan has recently added an extra 2.5 million people to the number it says are at risk of malnutrition

"This is the new face of hunger," Sheeran said. "There is food on shelves but people are priced out of the market. There is vulnerability in urban areas we have not seen before. There are food riots in countries where we have not seen them before."

WFP officials say the extraordinary increases in the global price of basic foods were caused by a "perfect storm" of factors: a rise in demand for animal feed from increasingly prosperous populations in India and China, the use of more land and agricultural produce for biofuels, and climate change.

The impact has been felt around the world. Food riots have broken out in Morocco, Yemen, Mexico, Guinea, Mauritania, Senegal and Uzbekistan. Pakistan has reintroduced rationing for the first time in two decades. Russia has frozen the price of milk, bread, eggs and cooking oil for six months. Thailand is also planning a freeze on food staples. After protests around Indonesia, Jakarta has increased public food subsidies. India has banned the export of rice except the high-quality basmati variety.

"For us, the main concern is for the poorest countries and the net food buyers," said Frederic Mousseau, a humanitarian policy adviser at Oxfam. "For the poorest populations, 50%-80% of income goes on food purchases. We are concerned now about an immediate increase in malnutrition in these countries, and the landless, the farmworkers there, all those who are living on the edge."

Much of the blame has been put on the transfer of land and grains to the production of biofuel. But its impact has been outweighed by the sharp growth in demand from a new middle class in China and India for meat and other foods, which were previously viewed as luxuries.

"The fundamental cause is high income growth," said Joachim von Braun, the head of the International Food Policy Research Institute. "I estimate this is half the story. The biofuels is another 30%. Then there are weather-induced erratic changes which caused irritation in world food markets. These things have eaten into world levels of grain storage.

"The lower the reserves, the more nervous the markets become, and the increased volatility is particularly detrimental to the poor who have small assets."

The impact of climate change will amplify that already dangerous volatility. Record flooding in west Africa, a prolonged drought in Australia and unusually severe snowstorms in China have all had an impact on food production.

"The climate change factor is so far small but it is bound to get bigger," Von Braun said. "That is the long-term worry and the markets are trying to internalise it."

The WFP is holding an emergency meeting in Rome on Friday, at which its senior managers will meet board members to brief them on the scale of the problem. There will then be a case-by-case assessment of the seriousness of the situation in the affected countries, before the WFP formally asks for an increased budget at its executive board meeting in June.

But the donor countries are also facing higher fuel and transport costs. For the biggest US food aid programme, non-food costs now account for 65% of total programme expenditure.