Tokyo Electric Power Co. said Friday that toxic water found to have leaked last August at one of the huge tanks at the accident-hit Fukushima No. 1 nuclear power plant was far more contaminated than initially announced.

After recalculating the radiation level, Tepco said the water contained 280 million becquerels per liter of beta ray-emitting radioactive materials such as strontium-90, instead of 80 million becquerels.

A total of 300 tons of toxic water was found to have leaked at that time, part of which is believed to have flowed into the adjacent Pacific Ocean. The Nuclear Regulation Authority assessed the severity of the incident to be level 3 on an eight-point international scale.

Tepco decided to review data on 173 water samples it took until last October, as it found readings may be lower than actual figures due to improper measurement formulas.

As for 104 samples, Tepco analyzed them again as it had kept them. But the utility did not have the remaining 69 samples, including the water that leaked, so it calculated the radiation level by using a theoretical formula.