Elevated levels of radiation have been found 125 miles from the power plant, which was destroyed by the March 11 earthquake and tsunami. That is well beyond the 18-mile exclusion zone that has been imposed.

Officials in the city of Tokamachi, in northwest Niigata Prefecture, detected 27,000 becquerels of radioactive cesium per kilogramme (2.2lbs) of waste in a school compost heap. By law, any waste containing just 8,000 becquerels per kg must be treated as radioactive waste.

Experts and residents say the government should have begun monitoring further afield immediately after the plant began leaking radioactivity.

‘Since the first week of the disaster, authorities have slowly been announcing that they would start checking fish, seaweed, vegetables for radiation,’ said Tom Gill, a British professor of anthropology at Meiji Gakuin University who is studying communities in the disaster zone.

‘And the response in each case has – quite reasonably – to ask why it wasn’t done previously,’ he said. ‘And this is no different.’

As well as being slow to broaden the monitoring, Mr Gill says the figures being provided by the authorities are ‘extremely inconsistent.’

The education ministry, charged with compiling data, says on its web site that the maximum level of radiation in Fukushima Prefecture […]