Wealthy U.S. individuals have already pulled most of their money from Swiss private banks and could exit altogether as a global clampdown on tax evasion and banking secrecy benefits onshore rivals, a report showed.

Boston Consulting Group (BCG) data showed U.S. clients have withdrawn almost completely from Swiss banks since 2006, particularly since an extended tax dispute between U.S. authorities and UBS (UBSN.VX)(UBS.N), Switzerland's largest bank.

North American assets held in Swiss private banks fell to just 2 percent of the total in 2010 from 18 percent just four years earlier, the BGC report showed on Tuesday. (read more)