Japan has decided it will start releasing radioactive water accumulated at the damaged Fukushima nuclear plant into the sea after treatment, infuriating local residents and worrying neighbouring countries.

The decision comes a decade after the nation’s worst-ever atomic disaster at the Fukushima Daiichi Nuclear Power Station.

Tokyo Electric Power is expected to start discharging the water into the sea in two years after it has been treated, the operator and the government said.

The operator has stored over 1.2 million tons of water in more than 1,000 huge tanks at the site. The operator said the space for tanks will be running out in 2022, though local officials and some experts say otherwise.

The plant suffered meltdowns at three of its six reactors after it was hit by a powerful earthquake and ensuing tsunami in March 2011.

Since then, the operator has continued to inject water into the three reactors to keep cooling melted atomic fuel there.

Radiation-contaminated water at the site has been treated through an advanced liquid processing system, but tritium - a radioactive isotope of hydrogen - cannot be removed from the water.

The government’s plan invited criticism and strong opposition at home and abroad.

Some NGOs including Greenpeace which  rejected the government’s plan to release radioactive water into the ocean and instead proposed mortar solidification technology and a land-based storage system, using large tanks.

Japan Fisheries Cooperatives raised concern about  the release as well.

Citizens' groups and some experts slammed the Japanese government for lacking explanation of the plan and consensus-building efforts.

South Korea called in Japanese Ambassador  and lodged a protest over Tokyo’s decision, Japan media reported.

China also expressed concerns on Monday in anticipation of the decision.