The agency also lowered the short-term rating to 'A-2' from 'A-1', with both ratings remaining "on CreditWatch with negative implications", it said, suggesting that they could be downgraded further.

Standard & Poor's decision, announced in a statement released Saturday Tokyo time, followed a similar move by rating agency Moody's.

The deadly earthquake and tsunami on March 11 damaged several TEPCO facilities, and the company now faces huge costs to contain the troubled Fukushima Daiichi nuclear plant, which continues to leak radiation.

"As a result of the unprecedented events of March 11 and the importance of TEPCO to the Japanese economy, we believe that the Japanese government may provide some form of extraordinary support to the company," S&P said.

"However, on a stand-alone basis, we view the credit profile of TEPCO as non-investment grade, with a continued high risk of negative rating transition in the short term," it said.