Shares of Lehman Brothers, a major player in the mortgage-backed securities market, plummeted nearly 40% Tuesday, prompting Standard & Poor's to place the brokerage's ratings, including its 'A' long-term and 'A-1' short-term counterparty credit ratings, on CreditWatch with negative implications.

"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," said Standard & Poor's. "Although the ratings ultimately could be affirmed, we do not currently rule out the possibility of lowering the ratings by more than one notch."

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