"China and India, the world's two largest producers and consumers of many agricultural commodities, already face severe water limitations in agricultural production," the Colombo-based scientific research group said.

"Domestic production of biofuels derived from crops will put greater stress on these countries' water supplies, seriously undermining their ability to meet future food and feed demands," it added.

It said China aimed to quadruple biofuel output to around 15 billion litres (3.30 billion Imp gallons) by 2020, or 9 percent of the nation's gasoline demand. To achieve that goal, China would have to raise maize output by 26 percent, it said.

India was reviewing similar biofuel targets to help offset global warming, widely blamed on greenhouse gases released by burning fossil fuels such as oil and coal, and raise domestic energy output. That would mean far more sugarcane plantings.

"They have to scale down considerably," said Charlotte de Fraiture, an IWMI scientist and lead author of the study. She gave some preliminary findings at a Stockholm conference in August.

The report did not take account of climate change that could disrupt rainfall and flows in many Asian rivers linked to a projected melting of Himalayan glaciers.

CELLULOSE

Alternatives to irrigated maize and sugarcane included developing new technologies that would exploit enzymes to break down cellulose, the woody walls of plants, into biofuels.

In the shorter term, nations could also exploit dry land rain-fed crops such as sweet sorghum, Jatropha or Pongamia. That could help small-scale farmers and curb rural poverty.

The report said that it took 2,400 litres of irrigation water to produce one litre of ethanol from maize in China. For the same amount of ethanol from Indian sugarcane, 3,500 litres of water was needed.

By contrast, it took just 90 litres of irrigation water to produce a litre of ethanol in Brazil from mainly rain-fed sugarcane.

Outside India and China, the study said that biofuels would only have a "modest impact" on water use and food systems around the world. Biofuels now account for only about 2 percent of annual gasoline output.