Japan's Environment Minister said yesterday he saw no choice but to discharge treated contaminated water from the wrecked Fukushima Daiichi nuclear power plant into the Pacific Ocean, as storage space is likely to run out by 2022.

This water, used to cool reactor cores, would have been treated to remove all radioactive contaminants except tritium, a low-toxic radioactive hydrogen isotope that is much harder to strip out.

Japan has long cited nuclear experts in stressing that the "tritiated water" is a routine by-product of nuclear power plants worldwide, and is commonly diluted and released into open waters without posing any risk to life or the environment.

But many remain unconvinced, given the toxic reputation linked to Fukushima since the nuclear meltdown of March 11, 2011.

South Korea has vehemently criticised the plan, while Fukushima fishermen yesterday called the idea "irresponsible".

Environment Minister Yoshiaki Harada, expected to be replaced in a Cabinet reshuffle today, told a news conference: "I believe there is no choice but to dump the water (into the ocean) and dilute it."

Chief Cabinet Secretary Yoshihide Suga said Mr Harada was expressing his "personal opinion". He stressed that no decision has yet been made and that the government is looking at all factors, "including social aspects like reputational damage".

Tokyo Electric Power, which runs the Fukushima plant, said it was not in a position to decide what to do but will follow the policy set by the government.

Tokyo is also considering four other options. These include encasing the water in cement and burying it, and vaporising the water into the air as steam.

The International Atomic Energy Agency has urged Tokyo to urgently reach a decision, though Japan is reluctant to draw attention to the issue before next year's Olympics, which it has branded the "reconstruction games".