The release is unlikely to begin for at least two years but has already sparked opposition from local fishing communities and concern in Beijing and Seoul.

Japan's government argues that the release will be safe because the water has been processed to remove almost all radioactive elements and will be diluted.

It has support from the International Atomic Energy Agency, which says the release is similar to processes for disposing of waste water from nuclear plants elsewhere in the world.

"The Japanese government has compiled basic policies to release the processed water into the ocean, after ensuring the safety levels of the water... and while the government takes measures to prevent reputational damage," Suga told reporters.

Around 1.25 million tonnes of water has accumulated at the site of the nuclear plant, which was crippled after going into meltdown following a tsunami in 2011.

It includes water used to cool the plant, as well as rain and groundwater that seeps in daily.

The water is pumped out and filtered, but the decision is certain to spark controversy, angering local fishing communities that have spent years trying 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."

The decision also prompted regional opposition even before it was official, with South Korea's foreign minister on Monday expressing "serious regret over this decision, which could have a direct or indirect impact on the safety of our people and the surrounding environment in the future".

Chinese foreign ministry spokesman Zhao Lijian urged Japan to "act in a responsible manner" over the discharge of the water.

"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 Monday.