Rice prices have surged to a 20-year high in the latest sign of global food inflation, creating policy headaches in Asia, where more than 2.5bn people depend on cheap and abundant supplies of the grain.

Thai rice prices, a global benchmark, surged last week above the level of $500 a tonne for the first time since at least 1989, according to the United Nations Food and Agriculture Organisation, prompting importing countries to seek assurances on supplies.