KBW analyst Diane Merdian now estimates Citigroup will take $15.3 billion in writedowns in the fourth quarter tied to mortgage-backed debt and collateralized debt obligations, instead of the $11 billion previously estimated.

The writedowns stem from continued deterioration in the mortgage market. As mortgages — especially subprime mortgages given to customers with poor credit history — have increasingly defaulted, banks have been forced to cut the value of bonds and debt backed by the troubled loans.

Citigroup took about $6 billion in writedowns during the third quarter. Late last year, Citigroup anticipated it would take between $8 billion and $11 billion in writedowns during the fourth quarter.

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