Three former executives of the crippled Fukushima nuclear plant operator were indicted Monday over the 2011 atomic accident, in what will be the first criminal trial linked to the disaster.

Ex-Tokyo Electric Power (TEPCO) chairman Tsunehisa Katsumata, and former vice presidents Sakae Muto and Ichiro Takekuro were formally charged with professional negligence resulting in deaths and injury for their role in the crisis.

The trio were not taken into custody.

"I'm full of emotion," Ruiko Muto, head of a campaign group pushing for a trial, told a Tokyo press briefing.

"This will be a great encouragement for hundreds of thousands of nuclear accident victims who are still suffering and facing hardship," she added.

A judicial review panel composed of ordinary citizens ruled in July -- for the second time since the accident -- that the three men should be put on trial.

The decision compelled prosecutors to press on with the criminal case under Japanese law.

Prosecutors had twice refused to press charges against the men, citing insufficient evidence and little chance of conviction.

It will be the first criminal trial over responsibility for the tsunami-sparked reactor meltdowns that forced thousands from their homes in the world's worst nuclear accident since Chernobyl in 1986.

It is expected to take at least six months for the first trial to start, said Yuichi Kaido, a lawyer representing the campaigner group.

The trio face jail time of up to five years in prison or a penalty of up to one million yen ($8,850) if convicted.

Public broadcaster NHK said the former executives would plead not guilty, arguing it was impossible to predict the size of the massive tsunami that slammed into Japan's northeast coast.

Although the March 11 earthquake and tsunami killed 18,500 people, the nuclear disaster it caused is not officially recorded as having directly killed anyone.

The charges are linked to the deaths of more than 40 hospitalised patients who were hastily evacuated from the area and later died.

Around a dozen others -- including TEPCO employees and members of Japan's Self Defense Forces -- were injured during the accident.

- 'Major step forward' -

Environmental group Greenpeace said the decision to press on with a criminal case was "a major step forward".

"The court proceedings that will now follow should reveal the true extent of TEPCO's and the Japanese regulatory system's enormous failure to protect the people of Japan," said Hisayo Takada, deputy programme director at the organisation's Japan office.

A report by the International Atomic Energy Agency last year said a misguided faith in the complete safety of atomic power was a key factor in the Fukushima accident.

It pointed to weaknesses in disaster preparedness and in plant design, along with unclear responsibilities among regulators.

A 2012 parliamentary report also said Fukushima was a man-made disaster caused by Japan's culture of "reflexive obedience", but no one has been punished criminally.

An angry public pointed to cosy ties among the government, regulators and nuclear operators that allegedly insulated TEPCO's executives from being charged.

Campaigners have called for about three dozen company officials to be held accountable for their failure properly to protect the site against the tsunami.

The accident at Fukushima forced the shutdown of dozens of reactors across Japan, with a handful now having been restarted.

Prime Minister Shinzo Abe and utility companies are still pushing to get reactors back in operation, nearly five years after the crisis.

But anti-nuclear sentiment remains high in Japan and there is widespread opposition to restarts.

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TEPCO - TOKYO ELECTRIC POWER