South Korea has suspended operations in two nuclear power reactors and extended a shutdown of a third to replace parts that were supplied using fake certificates.
The government warned on Tuesday there could be "unprecedented" electricity shortages and rolling blackouts this summer due to the nuclear shutdowns.
The latest closures, which are part of a widening investigation into the supply of cables provided with falsified safety certificates, mean that 10 of the country's 23 nuclear reactors are currently offline.
"Power shortages on an unprecedented scale are feared this summer," the Ministry of Trade, Industry and Energy said in a statement, adding that replacing suspect parts could take up to four months.
Blackout alerts, triggered automatically when power reserves dip below a certain level, were highly likely and power shortages could be "very serious" in August, the statement said.
The ministry said it would "strongly" enforce measures to reduce energy consumption, including rescheduling working hours to stagger demand and limit damaging peaks.
President Park Geun-hye expressed "great concern" about the prospect of power shortages and ordered a full, transparent probe into the case.
"Nuclear power [safety] is a very important issue linked directly with the safety of our people," Park told a cabinet meeting.
At proper capacity, South Korea's nuclear reactors supply more than 35 percent of national electricity needs.
"We expect unprecedented supply shortage this summer as we have to meet power demand while three reactors are halted," said Han Jin-hyun, vice minister for trade, industry and energy.
He added power saving measures would be unveiled this Friday.
The Nuclear Safety and Security Commission said it had shut down the reactors, one at the Gori nuclear complex and another at the Wolseong plant, after learning that both had used parts supplied with forged warranties.
The scheduled resumption of another reactor under maintenance at Gori, and the start of a new reactor at Wolseong, were postponed for the same reason, the commission said.
Parts used at all four reactors would have to be replaced, it added.
Last year officials found eight suppliers had faked warranties covering thousands of items used in a number of reactors.
Tuesday's ministry statement said further criminal and civil lawsuits would be filed against any organisation or individual found to have forged documents.
The case relates to forged documents on cables worth $5.35m provided in 2008, Kim and energy ministry officials said, declining to identify the cable producers.
The reactors, which each have a capacity of 1,000 megawatts (MW), will remain closed for about four months.