Merrill Lynch & Co Inc, which stunned Wall Street with $8.4 billion in write-downs in the third quarter, could have to write down another $4.5 billion in the current quarter, a Goldman Sachs analyst's report said on Thursday.

Merrill Lynch's remaining exposure to collateralized debt obligations and subprime mortgages stands at $20.9 billion, but the market for those securities continues to deteriorate. In a research note, Goldman Sachs said it was reasonable to assume a 20 percent write-down, or $4.5 billion, on those securities in the current fourth quarter.

On Thursday, Merrill Lynch senior executives did not rule out further write-downs during a conference call with analysts.