In November, the US's second-largest bank predicted possible investment write-offs of between $8 billion and $11 billion, following the US sub-prime crisis and tighter credit markets, which led to the resignation of chief executive Charles Prince.
However, Goldman Sachs is advising customers to sell Citigroup shares after predicting a $15 billion loss for the banking giant.
"We currently assume Citigroup will take an $11 billion write-off in the fourth quarter of 2007, at the high end of the firm's guidance, and we also assume an additional $4 billion write-off in the first quarter of 2008," its report said.
Citigroup have already announced writedowns to the tune of almost $2 billion.
Following deeper analysis, Goldman Sachs has revised its 2008 earnings forecast for Citigroup from $4.65 to $3.80 per share.