UBS has formally warned its American clients that U.S. tax authorities might be able to access records of their undeclared income in Switzerland following, according to a letter Reuters obtained.

The bank, which agreed to hand over 4,450 account names to the U.S. government in August, told clients they have 20 days to choose a lawyer in Switzerland, or be assigned one by the Swiss authorities, according to the Sept. 10 letter.

According to the letter, the process may "ultimately result in the submission of your account documents....to the IRS and the loss of the opportunity to participate in the IRS Voluntary Disclosure program.”

If individuals come voluntarily reveal income held in tax havens, they will pay back taxes, a lesser fine and generally avoid criminal prosecution. When that amnesty program expires, any tax cheats found by the government face higher fines and possibly criminal prosecution.

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