A Japanese court overturned a 13.3 trillion yen damages verdict on Friday against ex-bosses of the operator of the stricken Fukushima Daiichi nuclear plant, thought to be the largest such award ever in the country for a civil suit.

Four former executives had in 2022 been ordered to collectively pay 13.3 trillion yen in a suit brought by shareholders over the nuclear disaster triggered by a massive tsunami in 2011.

But the verdict was thrown out Friday by the Tokyo High Court, a spokeswoman for the institution told AFP.

Shareholders had argued the catastrophe could have been prevented if Tokyo Electric Power Co (TEPCO) bosses had listened to research and implemented preventative measures like placing an emergency power source on higher ground.

But the defendants countered that the risks were unpredictable, and the studies cited were not credible.

"The defendants... cannot be found to have had this foreseeability at a point in time before the earthquake in question," Friday's court ruling said.

The 13.3 trillion yen damages award was believed to be the largest amount ever ordered in a civil suit in Japan.

It was meant to cover TEPCO's costs for dismantling reactors, compensating affected residents, and cleaning up contamination.

The court spokeswoman said an appeal by the shareholders for an even higher damages order of 22 trillion yen had been denied.

"Take responsibility for the Fukushima nuclear accident!" said a pink-and-white banner displayed by the plaintiffs after the ruling.

Hiroyuki Kawai, head of their legal team, also issued a stark warning at a press conference on Friday.

"If I were to summarize today's ruling in one phrase: It is a ruling that will lead to future serious nuclear accidents," he said.

TEPCO declined to comment on the verdict.

Three of the Fukushima Daiichi nuclear plant's six reactors were operating when a massive undersea quake triggered a massive tsunami on March 11, 2011.

They went into meltdown after their cooling systems failed when waves flooded backup generators, leading to the worst nuclear disaster since Chernobyl.

Overall the tsunami along Japan's northeast coast left around 18,500 people dead or missing.

In March this year, Japan's top court said it had finalized the acquittal of two former TEPCO executives charged with professional negligence over the Fukushima meltdown.

The decision concluded the only criminal trial to arise from the plant's 2011 accident.