New figures show that this year's harvest will fail to produce enough to feed everyone on earth, for the sixth time in the last seven years. Humanity has so far managed by eating its way through stockpiles built up in better times, but these have now fallen below the danger level.

Food prices have already started to rise as a result, and threaten to soar out of reach of many of the 4.2 billion people who live in the 81 most vulnerable countries.

And the new drive to get cars to run on biofuels threatens to make food even more scarce and expensive.

The UN's Food and Agriculture Organisation (FAO) and the US Department of Agriculture (USDA), which produce the world's two main forecasts of global crop production, both estimate that this year's grain harvest will fall for the second successive year.

The FAO is still compiling its latest crop forecast, due to be published next month, but said it looked like barely exceeding 2,000 million tonnes, down from 2,038 million last year and 2,068 million in 2004, though the world's appetite has continued to grow as its population rises.

The USDA estimates it will be even lower, at 1,984 million tonnes. This would mean it would fall 58 million tonnes short of what the world's people are expected to consume this year: 10 years ago, by contrast, farmers grew 64 million tones more than was consumed.

Its figures show that production has met demand only once, in 2004, in the seven seasons since the start of the millennium. There has been no previous run of deficit years anything like this since 1960, when the USDA's figures began.

As a result, food stocks have fallen from enough to feed the world for 116 days in 1999 to a predicted 57 days at the end of this season. In the last year they have crashed through two vital barriers . . . 70 days, set by the FAO as the minimum safety level, and 60 days, after which scarcity starts rapidly driving up prices.

Lester Brown, president of Washington's respected Earth Policy Institute, says: "We are now living very close to the edge. Grain prices are already up 15% to 20% over earlier in the year. With oil already expensive, the world may soon be facing high grain and oil prices at the same time. This is a sobering prospect for the scores of poor countries that import both oil and grain. Many more people will go hungry, and die, and that will cause political instability."

The gathering crisis has been largely unnoticed because, for once, the harvests have failed in rich countries, which normally export food, like the US and Australia, rather than in the world's hungriest ones. So it has not immediately resulted in mass starvation in Africa or Asia.

Instead it will have a delayed effect as poor people increasingly become unable to afford expensive food, and there will be not enough in store to help them when their own crops fail.

The crisis is being made worse by the use of corn to make environmentally-friendly biofuels for cars, which this year will use up a fifth of the diminished US harvest.