The biofuels bonanza will crash unless producers can guarantee their crops have been produced responsibly, the UN's environment agency chief has said.

Achim Steiner of the UN Environment Programme (Unep) said there was an urgent need for standards to make sure rainforests weren't being destroyed.

Biofuel makers also had to show their products did not produce more CO2 than they negated, he told BBC News.

Critics say biofuels will lead to food shortages and destroy rainforests.

They point to the destruction of Indonesia's peat swamps as an example of biofuel folly.

The swamps are one the richest stores of carbon on the planet and they are being burned to produce palm oil.

Mr Steiner implied that because of Indonesia's inability to police its land use, biofuels from palm oil grown by the nation might never be deemed to be sustainable.

But he said some biofuels could be considered sustainable. He highlighted ethanol production in Brazil, and a dry land crop called jatropha, which is resistant to pests and droughts.

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Mr Steiner warns that Indonesia palm oil may never be sustainable

Mr Steiner urged investors not to turn their backs on developing second or third generation fuels that would use non-food crops and burnable waste.

He feared that beneficial biofuels might be lost as part of a consumer backlash.

Mr Steiner made his comments in response to criticism from a group of independent scientists who said they had written to the Intergovernmental Panel on Climate Change (IPCC) complaining that the climate body's comments on biofuels have been naive.

The independent scientists pointed to two phrases in reports by the IPPC, of which Unep is a co-sponsor, which the scientists said could not be substantiated.

One stated that biofuels were an effective solution in at least a number of countries, while the other suggested that biofuels in the transport sector would generally have positive social and environmental benefits.

False economy

One of the scientists, Tad Patzek from University of California Berkeley, US, said: "In the long-run, the planet cannot afford to produce biofuels because we're going to run out of the land and water and environmental resources.

"In addition, because of the land use changes, drying up peat-swamps, burning tropical forest, these biofuels involve up-front enormous emissions of greenhouse gases that will never be recouped by their later use," he told BBC News.

Professor Patzek also doubted Mr Steiner's confidence in Brazilian ethanol. "The [IPCC] description of Brazilian sugar-cane ethanol production as 'highly advanced' and 'a model' is somewhat of an exaggeration.

"It's neither good nor a model," he said.

Brazilian producers are adamant that their bio-crops are not grown on rainforest land - but the environmental group Friends of the Earth Brazil claim that peasant farmers - dispossessed by biofuel conglomerates - are moving to the Amazon to seek new land.

Mr Steiner said Brazil had enough land to ensure that biofuel cropping could be sustainable.

The group of scientists said their letter to the head of the IPCC, Professor Pachauri, had not been answered.

BBC News has not been able to obtain a comment from Professor Pachauri, though this may be hardly surprising given that the final summit on the IPCC Fourth Assessment Report (A4R) is currently underway in Valencia, Spain.

Mr Steiner said Unep had set up a high-level task force to study the life-cycle implications of all biofuels. The group is expected to publish its findings next year.

By then much of the Indonesian peat swamps - one of the most valuable stores of carbon in the world - will have been torched.

The only way of stopping may not be through the UN or the Indonesian government, but through one or more private philanthropist with a burning desire to own an Indonesian swamp.