Radioactive iodine at levels exceeding safety standards for infants was detected in water at a purification facility in Tokyo's Katsushika Ward on Wednesday.
Tokyo metropolitan government officials advised residents in a large part of the capital to refrain from using tap water to feed infants.
Officials said the iodine level was 210 becquerels per liter at the Kanamachi purification center, which serves Tokyo's 23 central wards, along with Musashino, Machida, Tama, Inagi and Mitaka cities.
The health ministry's standard issued Monday sets 100 becquerels as the maximum level considered safe for consumption by infants.
However, the officials also said the ministry standard is based on possible health risks caused by consumption over a prolonged period. They said the health risks are low in consuming tap water for the time being until alternative sources can be obtained.