It is the constant sensation of hunger that makes Kamla Devi so angry. She argues with shopkeepers in New Delhi over prices and quarrels with her husband, a casual labourer, over his wages - about 50 rupees (60p) a day.

'When I go to the market and see how little I can get for my money, it makes me want to hit the shopkeepers and thrash the government,' she says. A few months ago, Kamla - who is 42 - decided she and her husband could no longer afford to eat twice a day. The couple, who have already sent their two teenage sons to live with more prosperous relatives, now exist on only one daily meal. At midday Kamla cooks a dozen roti (a round, flat Indian bread) with some vegetables fried with onions and spices. If there are some left, they will eat them at night. The only other sustenance that the couple have are occasional cups of sugared tea.

'My husband and I would argue every night. In the end he told me it wouldn't make his wages grow larger. Instead we went down to one meal a day to cut costs.'

It is a grim, unsettling story. Yet it is certainly not an exceptional one. Across the world, a food crisis is now unfolding with frightening speed. Hundreds of millions of men and women who, only a few months ago, were able to provide food for their families have found rocketing prices of wheat, rice and cooking oil have left them facing the imminent prospect of starvation. The spectre of catastrophe now looms over much of the planet.

In less than a year, the price of wheat has risen 130 per cent, soya by 87 per cent and rice by 74 per cent. According to the UN's Food and Agriculture Organisation, there are only eight to 12 weeks of cereal stocks in the world, while grain supplies are at their lowest since the 1980s.

For the Devi family, and hundreds of millions of others like them, the impact has been calamitous, as Robert Zoellick, the World Bank President, warned at this weekend's G7 meeting in Washington. Brandishing a bag of rice, he told startled delegates from the world's richest nations that the world was now perched at the edge of catastrophe.

'This is not just about meals forgone today, or about increasing social unrest, it is about lost learning potential for children and adults in the future, stunted intellectual and physical growth,' he said. Without urgent action to resolve the crisis, he added, the fight against poverty could be set back by seven years.

Not surprisingly, these swiftly rising prices have unleashed serious political unrest in many places. In Dhaka yesterday 10,000 Bangladeshi textile workers clashed with police. Dozens were injured, including 20 policemen, in a protest triggered by food costs that was eventually quelled by baton charges and teargas. In Haiti, demonstrators recently tried to storm the presidential palace after prices of staple foods leaped 50 per cent.

In Egypt, Indonesia, Ivory Coast, Mauritania, Mozambique, Senegal and Cameroon there have been demonstrations, sometimes involving fatalities, as starving, desperate people have taken to the streets. And in Vietnam the new crime of rice rustling - in which crops are stripped at night from fields by raiders - has led to the banning of all harvesting machines from roads after sunset and to farmers, armed with shotguns, camping around their fields 24 hours a day.

But what are the factors that led to this global unrest? What has triggered the price rises that have put the world's basic foodstuffs out of reach for a rising fraction of its population? And what measures must be taken by politicians, world leaders and monetary chiefs to rectify the crisis? Not surprisingly, the first two of these questions tend to be the easier ones to answer. Economists and financiers point to a number of factors that have combined to create the current crisis, a perfect storm in which several apparently unconnected events come together with disastrous effects.

One key issue highlighted at the G7 meeting was the decision by the US government, made several years ago, to give domestic subsidies to its farmers so that they could grow corn that can then be fermented and distilled into ethanol, a biofuel which can be mixed with petrol. This policy helps limit US dependence on oil imports and also gives support to the nation's farmers. However, by taking over land - about 20 million acres so far in the United States - that would otherwise have been used to grow wheat and other food crops, US food production has dropped dramatically. Prices of wheat, soya and other crops have been pushed up significantly as a result.

Other nations, including Argentina, Canada and some European countries, have adopted similar, but more restrained, biofuel policies. But without mentioning any countries by name, Zoellick clearly pointed the finger of blame at the US. Everyone should 'look closely at the effects of the dash for biofuels', he said. 'I would hope that countries that, for whatever reason, energy security and others, have emphasised biofuel development will be particularly sensitive to the call to meet the emergency needs for people who may not have enough food to eat.'

This point has also been stressed recently by the UK government's chief scientific adviser, Professor John Beddington. 'It is very hard to imagine how we can see the world growing enough crops to produce renewable energy and at the same time meet the enormous demand for food,' he said. 'The supply of food really isn't keeping up.'

For his part, Hank Paulson, the US Treasury Secretary - asked about the impact of US energy policies on food prices on Friday - tried to bat away the question. 'This is a complex area, with a number of causes,' he told reporters. The first priority, he added, was to get food supplies to people who need them, before considering the longer-term reasons for the rising prices.

It was not a point shared by the chief of staff in the United Nations trade and development division, Taffere Tesfachew, who flew to London last week ahead of a vital meeting of the leaders of the world's poorest nations in Accra, the capital of Ghana. Instead of an agenda designed to achieve economic progress in the developing world, the meeting will instead focus on the pressing issue of food. Tesfachew said that decades of aid has been skewed to ambitious industrialisation programmes and that the World Bank and others have failed to invest in the agricultural sector. 'We believe these high food prices won't disappear in the next two years, so now is the time to redress imbalances in terms of ethanol subsidies,' he said.

Zoellick was also clear that action was now urgently needed. 'In the US and Europe over the last year we have been focusing on the prices of gasoline at the pumps. While many worry about filling their tanks, many others around the world are struggling to fill their stomachs. And it's getting more and more difficult every day,' added Zoellick, who made an impassioned plea to the world's rich nations to provide emergency help, including $500m in extra funding to the UN World Food Programme.

This call was backed by finance ministers from the G24, who represent the leading developing countries, who also demanded extra cash to help cushion the poor against the shock of rising food prices. As well as causing hunger and malnutrition, the rising cost of basic foodstuffs risks blowing a hole in the budgets of food-importing countries, many of them in Africa, they argued.

As to the other factors that have combined to trigger the current food crisis, experts also point to the connected issue of climate change. As the levels of carbon dioxide rise in the atmosphere, meteorologists have warned that weather patterns are becoming increasingly disturbed, causing devastation in many areas. For several consecutive years, Australia - once a prime grower of wheat - has found its production ruined by drought, for example. Scarcity, particularly on Asia's grain markets, has then driven up prices even further.

Some campaigners see climate change as the most pressing challenge facing the world while others now say that biofuels - grown to offset fossil fuel use - is taking food out of the mouths of some of the world's poorest people. The net result will be eco-warriers battling with poverty campaigners for the moral high ground.

On top of these issues, there is the growing wealth of China and its 1 billion inhabitants. Once the possessor of a relatively poor rural economy, China has becoming increasingly industrialised and its middle classes have swelled in numbers.

One impact has been to trigger a doubling in meat consumption, particularly pork. As the country's farmers have sought to feed more and more pigs, more and more grain has been bought by them. However, China has only 7 per cent of the world's arable land and that figure is shrinking as farmland has been ravaged by pollution and water shortages.

The net result has been to decrease domestic supplies of grain just as demand for it has started to boom. Again the impact has struck worst in the Third World, with wheat and other grain prices soaring.

And finally there is the issue of vegetable oils. Soya and palm oils are a major source of calories in Asia. But flooding in Malaysia and a drought in Indonesia have limited supplies.

In addition, these oils are now being sought as bio-diesel, which is used as a direct substitute for diesel in many countries, including Australia. The impact has been all too familiar: an alarming drop in supplies for the people of the Third World as prices of this basic commodity have soared.

One such victim is Kamla Devi. She has already had to abandon dhal, a central, protein-rich dish of lentils that was a key part of her family's diet for several months. Now the cooking of fried food - in particular, pooris: hot, puffed, oil-soaked bread - has had to follow suit for the simple reason that cooking oil has become unaffordable.

'It has affected my health,' she says. 'The rich are becoming richer. They go to shopping malls and they don't need to worry. The problem with prices only matters for the poor people like me.'

· Additional reporting by Amelia Gentleman and Nick Mathiasson

Four key factors behind the spreading fear of starvation across the globe

Growing consumption

Six months ago Zhou Jian closed down his car parts business and launched himself as a pork butcher. Since then the 26-year-old businessman's Shanghai shop has been crowded out - despite a 58 per cent rise in the price of pork in the past year - and his income has trebled.

As China's emerging middle classes become richer, their consumption of meat has increased by more than 150 per cent per head since 1980. In those days, meat was scarce, rationed at around 1kg per person per month and used sparingly in rice and noodle dishes, stir fried to preserve cooking oil.

Today, the average Chinese consumer eats more than 50kg of meat a year. To feed the millions of pigs on its farms, China is now importing grain on a huge scale, pushing up its prices worldwide.

Palm oil crisis

The oil palm tree is the most highly efficient producer of vegetable oil, with one acre yielding as much oil as eight acres of soybeans. Unfortunately, it takes eight years to grow to maturity and demand has outstripped supply. Vegetable oils provide an important source of calories in the developing world, and their shortage has contributed to the food crisis.

A drought in Indonesia and flooding in Malaysia has also hit the crop. While farmers and plantation companies hurriedly clear land to replant, it will take time before their efforts bear fruit. Palm oil prices jumped nearly 70 per cent last year, hitting the poorest families. When a store in Chongqing in China announced a cooking-oil promotion in November, a stampede left three dead and 31 injured.

Biofuel demand

The rising demand for ethanol, a biofuel that is mixed with petrol to bring down prices at the pump, has transformed the landscape of Iowa. Today this heartland of the Midwest is America's cornbelt, with the corn crop stretching as far as the eye can see.

Iowa produces almost half of the entire output of ethanol in the US, with 21 ethanol-producing plants as farmers tear down fences, dig out old soya bean crops, buy up land and plant yet more corn. It has been likened to a new gold rush.

But none of it is for food. And as the demand for ethanol increases, yet more farmers will pile in for the great scramble to plant corn - instead of grain. The effect will be to further worsen world grain shortages.

Global warming

The massive grain storage complex outside Tottenham, New South Wales, today lies virtually empty. Normally, it would be half-full. As the second largest exporter of grain after the US, Australia usually expects to harvest around 25 million tonnes a year. But, because of a five-year drought, thought to have been caused by climate change, it managed just 9.8 million tonnes in 2006.

Farmers such as George Grieg, who has farmed here for 50 years, have rarely known it to be so bad. Many have not even recovered the cost of planting and caring for their crops, and are being forced into debt. With global wheat prices at an all-time high, all they can do is cling on in the hope of a bumper crop next time - if they are lucky.

Food in figures

93,000,000 Acres of corn planted by US farmers last year, up 19 per cent on 2006.

76% Amount of US corn used for animal feed.

8kg Amount of grain it takes to produce 1kg of beef.

20% Portion of US corn used to produce five billion gallons of ethanol in 2006-07.

50kg Quantity of meat consumed annually by the average Chinese person, up from 20kg in 1985.

10% Anticipated share of biofuels used for transport in the EU by 2020.

$500m The UN World Food Programme's shortfall this year, in attempting to feed 89 million needy people.

9.2bn The world's predicted population by 2050. It's 6.6bn now.

130% The rise in the cost of wheat in 12 months.

16 times The overall food consumption of the world's richest 20 per cent compared with that of the poorest 20 per cent.

58% Jump in the price of pork in China in the past year.

$900 The cost of one tonne of Thai premier rice, up 30 per cent in a month.

Caroline Davies