NEW YORK (Reuters) -- GMAC LLC, the auto and mortgage lender controlled by Cerberus Capital, may face ""substantial difficulty"" if credit markets do not improve, Cerberus founder Stephen Feinberg wrote in a January letter to investors, Bloomberg News reported on Thursday.

""We have detailed contingency plans in a continuing worsening environment,"" Feinberg wrote, according to Bloomberg. ""However, if the credit markets continue to decline and we find ourselves in a prolonged environment of capital market shutdown, GMAC could run into substantial difficulty."" Cerberus declined to comment.
GMAC posted a $724 million fourth-quarter loss earlier this month and was downgraded by Moody's Investors Service after more customers fell behind on payments for their houses, cars and trucks.
The loss compared with a year-earlier $1.02 billion profit, but improved from a $1.6 billion loss in the third quarter.
Cerberus led a group that bought 51 percent of GMAC in 2006 from General Motors Corp (GM.N), which owns 49 percent of GMAC.