TOKYO — Three hundred tons of highly contaminated water has leaked from a storage tank at the ravaged Fukushima Daiichi Nuclear Power Plant on Japan’s Pacific coast, its operator said Tuesday, prompting regulators to declare a “radiological release incident” for the first time since disaster struck there in 2011 and adding new fears of environmental calamity.

Workers raced to place sandbags around the leaking tank to stem the spread of the water, contaminated by levels of radioactive cesium and strontium many hundreds of times as high as legal safety limits, according to the operator, Tokyo Electric Power, or Tepco. The task was made more urgent by a forecast of heavy rain for the region.

But a Tepco spokesman, Masayuki Ono, acknowledged that much of the contaminated water had seeped into the soil, which would have to be dug up and removed. And he said the tainted water could eventually reach the ocean, adding to the tons of radioactive fluids that have already leaked into the sea from the plant.

The new leak raises disturbing questions about the durability of the nearly 1,000 huge tanks Tepco has installed about 500 yards from the site’s shoreline. The tanks are meant to store the vast amounts of contaminated liquid created as workers cool the complex’s three damaged reactors by pumping water into their cores, along with groundwater recovered after it poured into the reactors’ breached basements.