Lawyers for UBS AG said the company would violate Swiss sovereignty and criminal law if it disclosed the names of 52,000 American customers.

A U.S. lawsuit filed Thursday improperly seeks to enforce summonses from the Internal Revenue Service for the identities of account holders and would trample on Swiss sovereignty, according to a UBS filing Friday in federal court in Miami.

"Swiss law strictly prohibits UBS and its employees from disclosing to the IRS the account information located in Switzerland that the IRS seeks," UBS lawyers wrote. "The IRS's petition does not acknowledge these restrictions and instead simply ignores the existence of Swiss law and sovereignty." The filing is UBS' first legal response to a lawsuit that would enhance tax collection by striking a blow at Swiss banks' secrecy. The U.S. sued one day after the Zurich company, Switzerland's largest bank, agreed to pay $780 million and disclose the names of about 250 customers to defer prosecution on a charge that it conspired to help wealthy Americans evade U.S. taxes over several years.

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