Tokyo Electric Power (Tepco) said initial reports of a level 10m times higher than normal in parts of the No 2 reactor were inaccurate, although it could not say by how much.

Tepco said at first that the worker who took the measurement, of a pool of water in the reactor's basement turbine building, had fled before taking a second reading. The discovery prompted another evacuation at the site, halting work to pump and store radioactive water that has built up in the turbine buildings of three of the six reactors.

Tepco later said the pool of water had been contaminated but the extremely high reading was a mistake. "The number is not credible," spokesman Takashi Kurita said. "We are very sorry."