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Biofuels have forced global food prices up by 75% - far more than previously estimated - according to a confidential World Bank report obtained by the Guardian.

The damning unpublished assessment is based on the most detailed analysis of the crisis so far, carried out by an internationally-respected economist at global financial body.

The figure emphatically contradicts the US government's claims that plant-derived fuels contribute less than 3% to food-price rises. It will add to pressure on governments in Washington and across Europe, which have turned to plant-derived fuels to reduce emissions of greenhouse gases and reduce their dependence on imported oil.

Senior development sources believe the report, completed in April, has not been published to avoid embarrassing President George Bush.

"It would put the World Bank in a political hot-spot with the White House," said one yesterday.

The news comes at a critical point in the world's negotiations on biofuels policy. Leaders of the G8 industrialised countries meet next week in Hokkaido, Japan, where they will discuss the food crisis and come under intense lobbying from campaigners calling for a moratorium on the use of plant-derived fuels.

It will also put pressure on the British government, which is due to release its own report on the impact of biofuels, the Gallagher Report. The Guardian has previously reported that the British study will state that plant fuels have played a "significant" part in pushing up food prices to record levels. Although it was expected last week, the report has still not been released.

"Political leaders seem intent on suppressing and ignoring the strong evidence that biofuels are a major factor in recent food price rises," said Robert Bailey, policy adviser at Oxfam. "It is imperative that we have the full picture. While politicians concentrate on keeping industry lobbies happy, people in poor countries cannot afford enough to eat."