The UN’s nuclear watchdog has approved plans by Japan to release more than 1m tonnes of water from the wrecked Fukushima Daiichi nuclear power plant into the Pacific Ocean, despite objections from local fishing communities and other countries in the region.

Rafael Grossi, the director general of the International Atomic Energy Agency (IAEA), said on Tuesday that the body’s latest safety review of the planned discharge “makes the science of the treated water release clear for the international community and it answers the technical questions related to safety that have been raised”.

The report said discharging the water would have “a negligible radiological impact on people and the environment”.

“This is a very special night today,” Grossi told the prime minister, Fumio Kishida, before handing him a blue folder containing the final report.

Kishida said Japan would “continue to provide explanations to the Japanese people and to the international community in a sincere manner based on scientific evidence and with high level of transparency”.

Grossi later said the document was “neither a recommendation nor an endorsement” of the water release plans drawn up by Japan’s government and the plant’s operator, Tokyo Electric Power (Tepco).

In a tweet, he said the IAEA would “continue our impartial, independent and objective safety review during and after the discharge phase”, adding that agency experts would “have a continuous on-site presence”.

The end of the two-year safety review brings Japan closer to the start of the long operation to pump the treated water – a mixture of groundwater, rain that seeps into the area, and water used for cooling damaged nuclear fuel – into the Pacific Ocean.

It is not clear when that process will begin, although there is speculation that it could be this summer.

About 1.3m tonnes of water stored in huge tanks on the site has been filtered through Tepco’s advanced liquid processing system (Alps) to remove most radioactive elements except for tritium, an isotope of hydrogen that is difficult to separate from water.

The “treated” water – Japanese officials object to the use of the word “contaminated” – will be diluted with seawater so that the concentration of tritium is well below internationally approved levels before being released into the ocean 1km from the shoreline via an undersea tunnel.

The water – enough to fill 500 Olympic-sized swimming pools – becomes contaminated when it is used to cool fuel rods that melted after the plant was hit by a powerful earthquake and tsunami in March 2011.

The disaster triggered a triple meltdown, in the world’s worst nuclear crisis since Chornobyl 25 years earlier.

Discharging the water is expected to take 30 to 40 years to complete, pending the IAEA’s review and official approval from Japan’s nuclear regulatory body, which could come as early as this week.

Attempts by Japanese government officials to win regional support for the plan have had limited success.

China, which denounced the plan as “extremely irresponsible” when it was announced in 2021, reiterated its opposition on Tuesday, calling for the discharge to be suspended.

Through its embassy in Japan, China said the IAEA’s report should not be interpreted as a “pass” for the water release.

Last week, a spokesperson for the country’s foreign ministry said Beijing urged Japan to “take seriously both international and domestic concerns, stop forcibly proceeding with its ocean discharge plan” and “subject itself to rigorous international oversight”.

Japan’s foreign ministry has said that it made multiple and repeated attempts to explain the science behind Tokyo’s stance to Beijing officials, but that its offers had been ignored.

Local Japanese fishing communities have also objected to the plan, saying it would destroy more than a decade of work rebuilding their industry, with shoppers likely to shun their catch and send prices plummeting.

Fukushima authorities have introduced some of the world’s strictest radiation testing regimes, but many consumers are yet to be reassured that fish and other produce from the region are safe.

The government and Tepco claim the environmental and health impacts from the water release will be negligible because the treated water will be released gradually after it has been diluted by large amounts of seawater.

Tritium is considered to be relatively harmless because it does not emit enough energy to penetrate human skin. But when ingested – via seafood, for example – it can raise cancer risks, a Scientific American article said in 2014.

The IAEA says nuclear plants around the world use a similar process to dispose of wastewater containing low-level concentrations of tritium and other radionuclides.

After visiting Fukushima Daichi on Wednesday, Grossi will travel to South Korea, where some people have been panic-buying sea salt over fears of contamination after the discharge begins.

Grossi is also expected to visit New Zealand and the Cook Islands in an effort to ease concerns over the plan, according to media reports.