Tokyo’s municipal government sparked a run on bottled water after warning that the city’s water supply had become contaminated with radiation, widening concerns about the impact on Japan’s food supplies from the Fukushima nuclear disaster.

Municipal officials said levels of iodine-131 measured at Tokyo’s water purifican plant had risen to 210 becquerels per kilogramme on Wednesday, more than twice Japan’s 100 bq/kg recommended limit for infants. The adult recommended limit is 300bq/kg.

Health officials played down the potential health risks from the contamination. Japan’s guidelines for iodine-131 in water are far tighter than the internationally agreed “operational intervention levels” for the substance, which put permissible levels at up to 3,000bq/kg, according to the World Health Organisation. CLIVE CHECKING WHAT IT IS FOR INFANTS