TOKYO — Japan’s nuclear watchdog said Wednesday that the crippled Fukushima power plant is probably leaking contaminated water into the ocean, a problem long suspected by experts but denied by the plant’s operator.
Officials from the Nuclear Regulation Authority said a leak is ‘‘strongly suspected’’ and urged plant operator Tokyo Electric Power Co. to determine where the water is leaking from and assess the environmental and other risks, including the impact on the food chain. The watchdog said it would form a panel of experts to look into ways to contain the problem.
The watchdog’s findings underscore Tokyo Electric’s delayed response in a problem that experts have long said existed. On Wednesday, the company continued to raise doubts about whether a leak exists.
Tokyo Electric spokesman Noriyuki Imaizumi said the increase in cesium levels in monitoring well water samples does not necessarily mean contaminated water from the plant is leaking to the ocean. Tokyo Electric was running another test on water samples and suspects earlier spikes might have been caused by cesium-laced dust slipping into the samples, he said. But he said Tokyo Electric is open to the watchdog’s suggestions to take safety steps.
The Fukushima Daiichi plant was ravaged by the March 2011 earthquake and tsunami and has since struggled with leaks of water used to cool the reactors, hampering decommissioning efforts.
Marine biologists have warned of the possibility of continuous leak of radioactive water into the sea via an underground water system, citing high levels of radioactivity in fish samples taken near the plant.
Since May, Tokyo Electric has reported spikes in cesium levels in underground water collected from a coastal observation pit, while the water-soluble element strontium showed high levels in seawater samples taken in areas off the coast of the plant. The firm says most of the contamination has been there since the 2011 accident.
Tokyo Electric has said it has detected ‘‘no significant impact’’ on the environment. It says cesium tends to be absorbed in the soil, and denies water contaminated with that element reached the sea.
But the Nuclear Regulation Authority said Wednesday that samples from both the pit water and coastal seawater shows tainted underground water probably had reached the sea.
Watchdog chairman Shunichi Tanaka said he thinks that the seawater contamination has been occurring since the accident, but that it was worst early in the crisis.
‘‘What’s most important is to minimize the leak to the outside and reduce the impact on the human society,’’ he said.