WASHINGTON — GMAC Financial Services, the consumer lender now majority owned by the government, said Tuesday it expects to report a loss of about $5 billion in the fourth quarter thanks to deep writedowns on underperforming mortgage loans it plans to sell.

The main driver of the anticipated loss is a $3.8 billion writedown taken as GMAC prepares to sell parts of a mortgage arm that CEO Michael Carpenter called "a millstone around the company's neck." GMAC received a $3.8 billion federal bailout last week — the third round of aid that now totals $16.3 billion. With those funds, the government's stake in GMAC is 56 percent, up from 35 percent. That could go to nearly 80 percent if the government opts to convert more of its stake to common equity.

It was only the latest helping hand the Obama administration has extended to GMAC, whose survival it sees as crucial to the rescue of automakers General Motors and Chrysler. The company provides wholesale financing to thousands of GM and Chrysler dealers, allowing them to stock their showroom floors.

In addition to the three rounds of financing, regulators have reluctantly allowed GMAC subsidiary Ally Bank to offer high interest rates that make its deposit base more volatile. And the company waited far longer than its larger competitors to acknowledge that its mortgage assets had lost much of their value.