Huge potential yield improvements from genetically modified foods threaten predictions that crop prices will remain high, the US Department of Agriculture's chief economist has warned.

Joseph Glauber, who is responsible for USDA forecasts, said that the department's central projection for the next decade or so was for "continuing high prices".

"Not continuing at last year's record levels, but still very strong levels," he told a conference in London, adding that demand for biofuels had set a floor for prices, with Chinese demand also a big factor.

'Very major threat'

However, this assumed yield increases of 1-1.5% per year, an assumption which could be tested by the next generation of genetically modified crops due on the market in three or four years' time.

If claims by seed companies were correct "we are going to see very large increases in yield", Mr Glauber told the World Agri Invest Congress.

"If we are moving from 150 bushels an acre to the 200s, 300s, as some claim we will, it will be of very major threat" to existing projections for farming.

Such yield increases would signal a return by crops to their long-running trend of falling prices, in real terms, he said.