The country plans to release into the sea more than one million tonnes of contaminated water from the destroyed Fukushima nuclear station, the government says.
Japan's government has decided to start releasing into the sea massive amounts of radioactive water stored in tanks at the wrecked Fukushima nuclear plant in two years after treatment.
Tuesday's decision, long speculated but delayed for years due to safety concerns and protests, came at a meeting of related ministers who endorsed the release as the best option.
The water has been accumulated and stored in tanks at the Fukushima Daiichi nuclear power plant since its 2011 meltdown after a massive earthquake and tsunami, causing cooling water to leak from the damaged reactors.
Its operator, Tokyo Electric Power Co., said its storage capacity will be full in the fall of 2022.
Impact on marine life
Prime Minister Yoshihide Suga told the ministers' meeting that the government adopted the release to sea as "utmost, realistic" and that the disposal of the water is "unavoidable in order to achieve Fukushima's recovery."
TEPCO and government officials say tritium, which is not harmful in small amounts, cannot be removed from the water, but all other selected radionuclides can be reduced to levels allowed for release.
Some scientists say the long-term impact on marine life from low-dose exposure to such large volumes of water is unknown.
Under a report of the basic plan adopted by the ministers on Tuesday, TEPCO will start releasing the water in about two years after building a facility under the regulatory authority's safety requirements.
'We can't back this move'
But local fishing communities fear releasing the water will undermine years of work to restore confidence in seafood from the region.
"They told us that they wouldn't release the water into the sea without the support of fishermen," Kanji Tachiya, who heads a local fisheries cooperative in Fukushima, told NHK ahead of the announcement.
"We can't back this move to break that promise and release the water into the sea unilaterally."
South Korea, China regret decision
The decision also prompted regional opposition even before it was official, with South Korea's foreign minister on Monday expressing "serious regret".
Chinese foreign ministry spokesman Zhao Lijian urged Japan to "act in a responsible manner".
"To safeguard international public interests and Chinese people's health and safety, China has expressed grave concern to the Japanese side through the diplomatic channel," Zhao said on Monday.
Greenpeace slams decision
Anti-nuclear activist group Greenpeace slammed Japan's government for having "once again failed the people of Fukushima".
"The cabinet's decision failed to protect the environment and neglected the large-scale opposition and concerns of the local Fukushima residents, as well as the neighbouring citizens around Japan," said climate and energy campaigner Kazue Suzuki in a statement.
The filtration process removes most radioactive elements from the water, but some remain, including tritium.
Experts say the element is only harmful to humans in large doses and with dilution the treated water poses no scientifically detectable risk.
"There is consensus among scientists that the impact on health is minuscule," Michiaki Kai, an expert on radiation risk assessment at Japan's Oita University of Nursing and Health Sciences, told AFP news agency before the decision was announced.