Japan is to begin releasing wastewater from the wrecked Fukushima Daiichi nuclear power plant from Thursday, in defiance of opposition from fishing communities, China and some scientists. The prime minister, Fumio Kishida, said on Tuesday he had asked the plant’s operator, Tokyo Electric Power (Tepco), “to swiftly prepare for the water discharge” in accordance with plans approved by nuclear regulators, adding that the release would begin on Thursday, “weather and ocean conditions permitting”. Kishida has said that disposing of more than 1m tonnes of water being stored at the site was an essential part of the long and complex process to decommission the plant. The plan has caused controversy because the water contains tritium, a radioactive substance that can’t be removed by the facility’s water filtration technology. Hong Kong, an important market for Japanese seafood exports, has threatened restrictions. Leader John Lee said on Tuesday he strongly opposed the water plan, adding that he had instructed the city’s government to “immediately activate” import controls on Japanese seafood. South Korea and China banned seafood imports from some areas of Japan after Fukushima Daiichi suffered a triple meltdown in the March 2011 triple disaster along the country’s north-east coast. China remains strongly opposed, accusing Japan of treating the ocean like a “sewer”. The South Korean government recently dropped its objections to the discharge, but opposition parties and many South Koreans are concerned about the impact the discharge will have on food safety. The decision comes weeks after the UN’s nuclear watchdog, the International Atomic Energy Agency (IAEA), approved the discharge, saying that the radiological impact on people and the environment would be “negligible”. Some experts point out that nuclear plants around the world use a similar process to dispose of wastewater containing low-level concentrations of tritium and other radionuclides. “Tritium has been released [by nuclear power plants] for decades with no evidential detrimental environmental or health effects,” said Tony Hooker, a nuclear expert from the University of Adelaide. Greenpeace, however, has described the filtration process as flawed, and warned that an “immense” quantity of radioactive material will be dispersed into the sea over the coming decades. Shaun Burnie, senior nuclear specialist at Greenpeace East Asia, said Japan’s government “has opted for a false solution – decades of deliberate radioactive pollution of the marine environment – during a time when the world’s oceans are already facing immense stress and pressures. “This is an outrage that violates the human rights of the people and communities of Fukushima, and other neighbouring prefectures and the wider Asia-Pacific region.” The government and Tepco also face opposition from local fishers, who say pumping water into the Pacific Ocean will destroy their industry. In a meeting on Monday with Masanobu Sakamoto, the head of the National Federation of Fisheries Cooperative Associations, Kishida attempted to reassure fishing communities that the discharge was safe. Ahead of the meeting, Sakamoto said his group’s opposition to the plan had “not changed one bit”. He said they understood that the water release could be scientifically safe, but still feared reputational damage. Those fears were echoed in a poll published this week by the Asahi Shimbun newspaper, in which 75% of respondents said the government had not done enough to prevent the expected reputational damage to Japanese seafood. Kishida acknowledged those concerns but insisted that the water release “is absolutely not something we can put off if we want to decommission the Fukushima nuclear plant and revive the area”. “I promise that we will take on the entire responsibility of ensuring the fishing industry can continue to make their living, even if that will take decades,” he told reporters on Monday. The government has set up funds worth ¥30bn ($206m) to compensate local fishers for reputational damage, and ¥50bn to address any financial impact on their business, according to the Kyodo news agency. The wastewater, which includes rain and groundwater, becomes contaminated when it is used to cool nuclear fuel rods damaged in 2011 when a powerful tsunami crashed into the plant, knocking out its backup electricity supply. About 1.3m tonnes of treated water – enough to fill 500 Olympic swimming pools – is being held in more than 1,000 steel tanks on the site, but Tepco has warned that storage space is running out. Tepco’s advanced liquid processing system removes most radioactive elements except for tritium, an isotope of hydrogen that is difficult to separate from water. The water will be diluted to one 40th of the concentration permitted under Japanese safety standards before being pumped into the ocean over the next 30 to 40 years via an underwater tunnel 1km from the coast. Tomoaki Kobayakawa, Tepco’s president, said the firm had a “strong determination to prevent reputational damage” during the discharge period. Tepco, he added, will “devote all of its resources to ensuring the safety and quality of facility operation, speedily obtaining monitoring results and disseminating that information in an accurate and easy-to-understand manner”. The water will be released at a maximum rate of 500,000 litres a day. A Japanese official said the first results of tests on discharged seawater could be available at the start of next month. Japan will also test fish in the waters near the plant, and make the results available on the agriculture ministry’s website.