Unit 2 reactor at the Fukushima plant was evacuated and officials said the recovery effort was being hampered by the high radiation levels. But earlier readings of 10 million times more than normal were false.

The alert came as officials acknow­ledged there was radioactive water in all four of the complex’s reactors.

Airborne radiation in Unit 2 measured 1,000 millisieverts per hour – four times the limit deemed safe by the government.

Experts still don’t know where the radioactive water is coming from, though government spokesman Yukio Edano said some was “almost certainly” seeping from a cracked reactor core in one of the units.

The plant’s operator Tepco said last night: “High levels of caesium and other substances are being detected, which usually should not be found in reactor water. There is a high possibility that fuel rods are being damaged.”