The virus, on a computer belonging to an employee of Mitsubishi Electric Plant Engineering (MPE), distributed the reports to peer-to-peer file sharing system Winny.
Officials said the leak occurred when the 30-year-old employee used his personal computer for company business. The data leaked included pictures of power plants and names and addresses of inspecting engineers.
The Japanese government's Chief Cabinet Secretary Hiroyuki Hosoda, acknowledged the data leakage and told Japanese media an investigation has been launched, but it has not been determined whether there has been "a leak of critical, important information concerning protection of nuclear materials".
"The fall-out from this breach acts as an unpleasant reminder that all businesses need to take computer security seriously," said Graham Cluley, senior technology consultant at antivirus company Sophos. "If you allow your employees to put sensitive company data onto their own home computers, you are running the risk that they will not be as well defended as the PCs within your organisation."
Sophos is also warning of a new computer virus that hides behind false news stories about the capture of Osama Bin Laden or conspiracy theories about Pope John Paul's death. The Kedebe.F worm disables security software and sends itself to other computers.
"Hackers are constantly trying to dupe computer users into running malicious code with the promise of breaking news stories," said Cluley. "Using the late Pope's name is a sick trick designed to fool the unwary. Everyone should exercise extreme caution, run up-to-date anti-virus software, and ensure they never run unsolicited email attachments."