Twelve years after one of the world's worst nuclear accidents, on August 24 Japan began releasing treated cooling water from the facility into the Pacific Ocean.

Many Japanese fishermen have been against the release, fearing that it will undo years of efforts to improve the industry's image in the wake of the 2011 catastrophe.

More than 100 plaintiffs, including fishermen in Fukushima and neighbouring prefectures, will file the lawsuit in the Fukushima District Court on Friday, Sugie Tanji, who is a member of the group's secretariat, told AFP.

"The government failed to keep to its promise of gaining agreement from fishermen before taking such a decision to release," she said.

"This is a wrong policy as it ignores strong opposition from not only the Fukushima fishermen's cooperative but also from cooperatives across the country," the group said in a statement.

"The release to the ocean can never be tolerated as it brings about further suffering to victims of the nuclear accident," it added.

The release has generated a fierce backlash from China, including a blanket ban of Japanese seafood imports.

Japanese government officials have made efforts to appeal to the public that the action has little impact on health or safety issues.

Last week, Prime Minister Fumio Kishida and the US envoy to Japan, Rahm Emanuel, among others, ate Fukushima fish in front of TV cameras.

Japan has repeatedly insisted the wastewater is treated and is harmless, a position backed by UN atomic watchdog the International Atomic Energy Agency (IAEA).

In total, plant operator TEPCO plans to release around 540 Olympic swimming pools' worth of water over the next several decades.