By Tenzin Pema

BANGALORE (Reuters) - Wall Street research analysts are projecting yet another tough quarter for U.S. investment banks marked by additional writedowns across a series of fixed-income assets amid an already weak operating environment.

Shares of Lehman Brothers Holdings LEH.N fell as much as 9 percent in morning trade Thursday, after a Citigroup analyst forecast big losses for the fourth largest U.S. investment bank, while a newspaper reported that an intended asset sale had collapsed.

Citigroup analyst Prashant Bhatia widened his third-quarter loss estimate for Lehman Brothers Holdings LEH.N. Bhatia and Lehman Brothers Inc analyst Roger Freeman also cut their third-quarter earnings estimates for Goldman Sachs Group (GS.N: Quote, Profile, Research) and Morgan Stanley (MS.N: Quote, Profile, Research).

"We are lowering our third-quarter estimates to reflect the difficult operating environment, characterized by lower client-related trading volumes and losses on hard-to-sell assets," Citigroup's Bhatia said.

Bhatia expects Lehman to take fresh asset-related writedowns of $2.9 billion (1.54 billion pounds). He expects $1.8 billion in writedowns at Goldman and $1.7 billion at Morgan Stanley.

"Based on further deterioration in several indices, we expect further writedowns, primarily related to mortgage assets," Bhatia wrote in an August 20 note to clients.

Lehman analyst Roger Freeman said writedowns for this quarter would come from exposures to prime residential mortgages, subprime mortgages and securities, Alt-A residential mortgages and securities, collateralized debt obligations, and leveraged loans.

Commercial mortgages have also experienced price depreciation this quarter, as measured by widening commercial mortgage-backed securities spreads and a decline in the CMBX index, which tracks commercial-mortgage-backed securities, Freeman said.