Shares of UBS fell sharply Friday following warnings from Citigroup analysts that the Swiss banking giant might face an additional writedown of up to $18 billion in 2008.
The stock closed 3.8 percent lower. Shares have lost more than have their value since the start of June.
UBS already wrote down $18 billion in 2007 due to its exposure to risky U.S. mortgages and linked assets, and called on shareholders to approve an emergency capital hike later this month.
"We estimate that the $80 billion remaining exposures could need 12-20 billion francs more markdowns in 2008," said the Citigroup note, which was published on Thursday.
A UBS spokeswoman said "our exposures are disclosed," but declined to comment on speculation of further write-downs to come.
Societe Generale cut its price target on the stock to 32 francs from a previous 40, while traders speculated the shares could bottom out at 25 francs.
Investment bank Exane BNP cut its investment rating on the bank's shares to "underperform" from "neutral" and set a price target of 45 francs.
The new downgrades follow a spate of similar downgrades that came in the wake of UBS's announcement on Thursday.
"Traders expect the shares will fall to CHF 25 in the near future," said one note circulating among Zurich traders.
But some market watchers disagreed with the assessment of Citigroup analysts.
"If the market deteriorates further there definitely would be more writedowns, but not only at UBS," Georg Kanders, analyst from WestLB Research, told CNBC.com.
UBS' results were very positive from an investors' perspective, Ralph Silva, analyst from Tower Group, told CNBC.com. Silva believes the bank's previous writedowns could have been much worse and doesn't expect any major negative surprises from UBS.