By John Letzing 

ZURICH--The market value of Switzerland's biggest banks crumbled after a surprise decision by the central bank to do away with a cap on the country's currency.

The Swiss National Bank's move Thursday to ditch a long-standing policy of maintaining a limit of 1.20 Swiss francs per euro suddenly exposed the country's big banks to relative declines in the value of both the euro and U.S. dollar--currencies in which lenders including UBS AG and Credit Suisse Group AG derive much of their earnings. As European trading ended Thursday, the euro had fallen to about 1.05 francs. The U.S. dollar also fell sharply relative to the franc.

The central bank had pledged for years to maintain the cap to curb the value of the franc, keep Swiss-made products affordable and facilitate Switzerland's export-driven economy. Another potential negative: The bank said it would raise the rate charged on overnight deposits by the big lenders that exceed a certain amount.

The hundreds of smaller, private banks that dot Switzerland--already under duress--may also suffer from a potential decline in dollar- or euro-denominated income. They will have fewer francs to cover their costs when they repatriate their earnings.

Meanwhile, the portfolios they maintain for wealthy clients could suffer from declines in the value of those currencies.

"It's bad for the whole banking sector," said Andreas Ruhlmann, an analyst at IG Bank.

Shares of Zurich-based UBS, the country's biggest bank, fell nearly 12%. Credit Suisse's shares were down almost 11%. The stock moves shaved a combined 11.7 billion francs ($11.5 billion) from the two banks' total market value, nearly equivalent to the gross domestic product of Albania.

Representatives for the two banks declined to comment.

The Swiss Bankers Association, the industry's lobbying group, said in a statement that it expects the SNB to now actively prevent a severe run-up in the value of the franc. Lombard Odier Group, the closely held, Geneva-based private bank where Patrick Odier, chairman of the Swiss Bankers Association, serves as senior managing partner, declined to comment through a spokesman.

According to Barclays PLC research, UBS is expected to derive 75% of its profit next year in currencies other than the franc, while Credit Suisse is pegged to make 80% of its profit in nonfranc currencies.

Credit Suisse has said previously that 27% of its total expenses during the first three quarters of last year were in francs, compared with 19% of its net revenue in the period.

Analysts at Morgan Stanley said in a note that the SNB's move may shave as much as 24% from Credit Suisse's earnings per share, and as much as 16% from those reported by UBS. The analysts added that they are now more certain that Credit Suisse will cut its 2014 cash dividend from the 70 rappen, or hundredths of a franc, designated for the prior year.

In addition to dropping its currency cap, the SNB said it is raising the rate charged on deposits from banks held overnight to 0.75% from the 0.25% level established last month.

Both big banks are required to hold a certain amount of funds at the central bank, though the lenders said last month that their deposits weren't above a threshold that would trigger the charges.

The SNB said Thursday that it is maintaining the current threshold, which is 20 times the minimum amount each bank is required to place on deposit.

While smaller, private Swiss banks may be less affected by the new deposit rate, they may suffer more from the SNB's scrapping of the currency cap, according to Barclays. A larger proportion of their operations and costs are denominated in francs, while managed assets and revenues are largely in dollars or euros.

Julius Baer Group AG, a Zurich private bank, may derive 87% of its profit next year in currencies other than the franc, according to Barclays.

Shares of Julius Baer fell more than 11%. A spokesman declined to comment.

Write to John Letzing at john.letzing@wsj.com

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