Radioactive iodine dropped back below the level safe for infants in Tokyo drinking water on Thursday, an official of the Japanese capital city's government said, confirming media reports.

In one Tokyo ward, Katsushika, a water sample that was taken on Tuesday and publicised on Wednesday had contained more than double the legal limit for infants, at 210 becquerels per kilogramme.

But the level fell back to 79 in a test Thursday, a Tokyo official told AFP.

The upper limits are 100 becquerels for infants and 300 for older people.