The key informant in the U.S. tax evasion case against Swiss bank UBS AG faces prison next year, but his harsher-than-expected treatment by the U.S. Justice Department will undermine efforts to expose secretive offshore tax havens, lawyers and whistle-blower advocates say.

Bradley Birkenfeld, a 44-year-old U.S. citizen, has been hailed by his attorneys and prosecutors alike as pivotal to the tax case against UBS, his former employer.