The decision was announced by activists supporting the prosecution of the three men, following the appeal hearing in the only criminal trial to arise from the world's worst nuclear accident since Chernobyl.

The court declined to comment on the verdict while the session was ongoing.

A massive tsunami swamped the Fukushima Daiichi plant on Japan's northeastern coast in March 2011 after a 9.0-magnitude undersea earthquake, the strongest in the country's recorded history.

The tsunami left 18,500 people dead or missing, but no one was recorded as having been directly killed by the nuclear accident, which forced evacuations and left parts of the surrounding area uninhabitable.

Wednesday's ruling affirmed a non-guilty verdict in September 2019 for the ex-bosses from the Tokyo Electric Power Company (TEPCO).

The three men had faced up to five years in prison if convicted, standing accused of liability for the deaths of more than 40 hospitalised patients who had to be evacuated following the nuclear disaster.

But the Tokyo District Court said in 2019 that they could not have predicted the scale of the tsunami that triggered the disaster, a decision that was upheld by the High Court.

The criminal case has been in the spotlight after a separate landmark verdict in July in a civil case involving the same three men and one other former executive.

The four were ordered to pay a whopping 13.32 trillion yen ($101 billion at today's rates) for failing to prevent the disaster.

Lawyers have said the enormous compensation sum is believed to be the largest amount ever awarded in a civil lawsuit in Japan -- although they admit that is symbolic, as it is well beyond the defendants' capacity to pay.