The ruins of the hard-hit Namie in Fukushima, Northeast Japan Photo: VCG

A Japanese court on Thursday cleared three former executives of professional negligence in the only criminal trial stemming from the 2011 Fukushima nuclear meltdown.

The three men were senior officials at the TEPCO firm operating the Fukushima Daiichi plant and had faced up to five years in prison if convicted.

"All defendants are not guilty," the presiding judge announced to the ­Tokyo courtroom, where a woman in the gallery could be heard saying "unbelievable" as the verdict was pronounced.

The decision is likely to be appealed, extending the legal wrangling over responsibility for the disaster, more than eight years after the worst nuclear accident since Chernobyl.

Outside the courtroom, dozens of people staged a rally, including some who had travelled from the Fukushima region to hear the verdict.

"It is absolutely an unjust ruling. We absolutely cannot accept this," one woman said angrily, addressing the crowd.

"We will appeal this and continue our fight," shouted a man nearby.

The three former executives were accused of professional negligence resulting in death and injury for failing to act on information about the risks from a major tsunami, but they argued the data available to them at the time was unreliable. The presiding judge said the verdict had turned on the "predictability" of the massive tsunami that swamped the nuclear plant in March 2011 after a 9.0-magnitude undersea earthquake.

No one was killed in the nuclear meltdown, but the tsunami left 18,500 dead or missing.

The ex-TEPCO executives faced trial in relation to the deaths of more than 40 hospitalized patients who died after having to be evacuated following the nuclear disaster.

Prosecutors twice declined to proceed with the case, citing insufficient evidence and a slim chance of conviction, but were forced to proceed after a judicial review panel composed of ordinary citizens ruled that the trio should face trial.