U.S. clients of UBS AG have been warned by the bank that their undeclared income in Switzerland may be revealed to U.S. tax authorities, according to a letter obtained by Reuters on Friday.

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

The process may ultimately result in the submission of your account documents .... to the IRS (U.S. Internal Revenue Service) and the loss of the opportunity to participate in the IRS Voluntary Disclosure program, the letter said.

As part of that U.S. amnesty program, which ends September 23, individuals pay back taxes, a reduced fine and can avoid criminal prosecution.

(Reporting by Kim Dixon; Editing by Lisa Von Ahn)