Shin Kori nuclear reactors 3 and 4 were supposed to be operational by next August or September, but Wednesday's findings mean the new starting date will be pushed back by up to a year.
Control cables, supplied by JS Cable, failed to pass quality assurance tests, which were conducted after the Nuclear Safety and Security Commission received a tip in late April that fake test reports had been used in the reactors.
The control cable is a device that sends a signal to the safety system in the event of an accident at the plant.
The parts in question will have to be replaced - a process which is expected to take six months to one year.
"We will use all our resources to replace the problematic parts safely and promptly."
Shin Kori reactors 3 and 4 have the capacity of 1.4 million kilowatts each.
Experts say the postponement will create a more acute power shortage crisis next year than the one the country went through this past summer.
Nearly half of the nation's 23 nuclear power plants have been shut down for one reason or another, and this has left a power vacuum of some three million kilowatts.
Power authorities project the disqualification of control cables used in Shin Kori reactors 3 and 4 will inflict damage of some three trillion won, or 2.8 billion U.S. dollars, per year on the Korean economy.
This is because it is much more expensive to use coal and liquefied natural gas to make up for the power loss every day.
Kim Yeon-ji, Arirang News.