Japan will start releasing treated radioactive water from the tsunami-hit Fukushima nuclear plant into the Pacific Ocean on Thursday, despite opposition from its neighbours.

The decision comes weeks after the UN’s nuclear watchdog approved the plan.

Some 1.34 million tonnes of water – enough to fill 500 Olympic-size pools – have accumulated since the 2011 tsunami destroyed the plant.

The water will be released over 30 years after being filtered and diluted.

Authorities will request for the plant’s operator to “promptly prepare” for the disposal to start on 24 Aug if weather and sea conditions are appropriate, Japan’s Prime Minister Fumio Kishida said after a Cabinet meeting on Tuesday.

Mr Kishida had visited the plant on Sunday, prompting speculation that the release was imminent.

The government has said that releasing the water is a necessary step in the lengthy and costly process of decommissioning the plant, which sits on the country’s east coast, about 220km (137 miles) north-east of the capital Tokyo.

Japan has been collecting and storing the contaminated water in tanks for more than a decade, but space is running out.

The 2011 tsunami, triggered by a magnitude 9.0 earthquake, is regarded as the world’s worst nuclear disaster since Chernobyl.

Shortly after, authorities set up an exclusion zone which kept getting expanded as radiation leaked from the plant, forcing more than 150,000 people to evacuate from the area.

The plan to release water from the plant has caused alarm across Asia and the Pacific since it was approved by the Japanese government two years ago.

It had been signed off by the UN’s nuclear watchdog, which concluded that the impact on people and the environment would be negligible, but fishermen in the region still fear that discharging the treated water will tarnish the reputation of their catch and affect their livelihoods.

Tepco has been filtering the water to remove more than 60 radioactive substances but the water will not be entirely radiation-free as it will still contain tritium and carbon-14, radioactive isotopes of hydrogen and carbon respectively that cannot be easily removed from water. But experts believe they are not a danger unless consumed in large quantities, because they emit very low levels of radiation.

The plan has caused uproar in neighbouring countries, with China the most vocal opponent. It accused Japan of treating the ocean like its “private sewer.”

South Korea, however, has endorsed the plan, and has accused protesters of scaremongering.

People who live in and around Fukushima are also not convinced that the treated water will be safe and many fishermen in the country fear that the release will affect their livelihoods.

Both South Korea and China have banned fish imports from around Fukushima.