Around 120 tons of contaminated water with an estimated 710 billion becquerels of radioactivity has probably leaked into the ground under the Fukushima No. 1 power plant, Tokyo Electric Power Co. revealed Saturday.

“It is the largest amount of radioactive substances that has been leaked” since the crippled facility’s cold shutdown was declared in December 2011, Tepco official Masayuki Ono said.

The utility, which announced the leak overnight, said Saturday morning that the water escaped from one of seven underground reservoir tanks at the No. 1 plant and that the remainder — an enormous 13,000 tons — is being pumped to other tanks nearby.

Although the process is likely to be completed early this week, Tepco warned that up to 47 tons of the highly irradiated water may additionally leak out before the task is completed. As the tank will out of commission for some time while the incident is investigated, Tepco is also looking to secure a new storage facility for the radioactive water.

The tank in question held processed water that had been used to cool down the plant’s stricken reactors. Able to hold about 14,000 tons of water, the tank, which Tepco began using to store contaminated water in February, almost reached full capacity last month.

The utility believes the radioactive water may have leaked out through a joint in the tank’s seepage control sheets.

Although much of the cesium had been removed, the water was still tainted with other dangerous radioactive substances. According to the utility, around 710 billion becquerels of radioactive materials are estimated to have seeped out of the tank, which measures around 60 meters long, 53 meters wide and 6 meters deep and is covered by three layers of waterproof sheeting.

Water found around the tank also turned out to be radioactive, the utility said.

Between the waterproofing sheets, it was measured at around 6,000 becquerels per cubic centimeter Friday, according to Tepco and the Nuclear Regulation Authority. Trace amounts of radioactive material were also detected in water between the outermost sheet and the soil.

The tank is around 800 meters from the Pacific but Tepco said it believes the irradiated water is unlikely to make its way to the sea.