Sept. 9 (Bloomberg) -- Lehman Brothers Holdings Inc., the fourth-biggest U.S. securities firm, was put on watch for a possible downgrade to its credit rating by Standard & Poor's as the shares plunged 45 percent today.

Lehman's long-term `A' and short-term `A-1' counterparty credit ratings were put on credit watch with ``negative'' implications, S&P said today.

``The CreditWatch listing stems from heightened uncertainty about Lehman's ability to raise additional capital, based on the precipitous decline in its share price in recent days,'' S&P analyst Scott Sprinzen wrote. S&P said it doesn't rule out the possibility of ``lowering the ratings by more than one notch.''

Lehman may incur a ``substantial'' net loss in the third quarter tied to writedowns of mortgage-related investments, Sprinzen said. The New York-based firm is seeking to raise capital and sell devalued real-estate assets after $8.2 billion in writedowns and credit losses in the past year.

Lehman declined $6.36 to $7.79 as of 4:01 p.m. in New York Stock Exchange composite trading.

Lehman's second-quarter loss of $2.8 billion was its first as a publicly traded company. Analysts surveyed by Bloomberg expect the firm to report a $2.2 billion third-quarter loss next week.

Capital-raising needs may be hurt if the company continues efforts to try to sell certain assets, including its commercial real-estate portfolio, S&P said.

``We continue to view Lehman's near-term liquidity as satisfactory,'' Sprinzen said. ``Overall, and despite nervous market sentiment in recent months, Lehman has maintained a very stable funding profile. We consider its excess liquidity position and contingent funding plan to be sound.''

Lehman could meet its secured funding needs by drawing on credit facilities extended by the U.S. Federal Reserve since Bear Stearns Cos. ``near collapse'' in mid-March, he said.

To contact the reporter on this story: Josh Fineman in New York at jfineman@bloomberg.net

Last Updated: September 9, 2008 16:17 EDT