The water, found in an underground tunnel linked to the number two reactor at the Fukushima plant, showed a radiation reading of more than 1,000 millisieverts per hour, a Tokyo Electric Power official told reporters.
There is estimated to be several thousand cubic metres of water inside the tunnel.
“We need to check if the water could flow directly into the sea,” he said.
Each reactor turbine building is connected to a maintenance tunnel large enough for workers to walk through.
In the case of the reactor two tunnel, the water has risen to within one metre of the top of a 15.9-metre access shaft which is located 55 metres from the sea, raising concerns it could overflow.
Levels of radioactive iodine some 1,850 times the legal limit were reported on Sunday several hundred metres offshore, but officials ruled out an immediate threat to marine life and seafood safety.
Radiation of more than 1,000 millisieverts per hour was earlier found in water in the turbine building of the number two reactor, delaying urgent work to restore the cooling systems and bring the facility back under control.
The exposure limit set by the government for workers at the tsunami-stricken plant is 250 millisieverts per year. A single dose of 1,000 millisieverts can cause temporary radiation sickness, including nausea and vomiting.
Workers are trying to pinpoint the exact source of the radioactive water leak, but there are concerns that fuel rod vessels or their valves and pipes are damaged.
A March 11 magnitude 9.0 earthquake and towering tsunami knocked out cooling systems for the six reactors at the Fukushima plant, leading to suspected partial meltdowns in three of them.
Huge amounts of seawater has been hosed onto the plant in a bid to keep the fuel rods from being exposed to the air, and prevent a full meltdown.
The tunnels for reactors one and three are also close to overflowing, but so far workers have not detected such high radiation levels in the water in those, the official said.
Nuclear safety agency spokesman Hidehiko Nishiyama said the operator would focus on trying to pump water out of the turbine buildings, which it hopes will ease the problem with the water in the tunnels.