Around 1.34 million tons of water, equivalent to more than 500 Olympic swimming pools, have accumulated since the Fukushima plant was knocked out by an earthquake and tsunami that killed 18,000 people in 2011.

Plant operator TEPCO says that with around 1,000 steel tanks now full, space has run out and that it wants to gradually start discharging the water into the Pacific via a one-kilometer (half-a-mile) underwater pipe.

"We would like to hold a meeting of the relevant ministers tomorrow (Tuesday) in order to make a decision on the commencement of the water release after confirming the status of efforts to ensure safety and to address reputational damage," Yasutoshi Nishimura, economy, trade and industry minister, told a news conference on Monday.

"Relevant ministers will discuss and share information on what next steps should be taken, and based on these discussions, we would like to make a decision about the timing," he said.

A TEPCO official said at a separate news conference that, once the government decision is taken, the release would begin "one to two days" later.

The government had said it planned to begin releasing the water before the end of the summer.

Diluted and filtered

The water has collected in the past 12 years from water used to cool three melted-down reactors, combined with groundwater and rain at the site in north-east Japan.