Standard & Poor's said on Friday it may cut its ratings on the preferred shares and subordinated bonds of Fannie Mae and Freddie Mac, citing concerns that U.S. government plans to shore up the mortgage finance companies may subordinate the debt.

The senior debt of the government sponsored enterprises (GSEs) was affirmed at the top "AAA," due to the government's explicit and implicit support of the companies. Their risk to the government ratings were also placed under review for downgrade.