By Tom Brown

MIAMI, Jan 4 (Reuters) – A U.S. judge on Monday upheld the prison sentence of a key informant in the tax fraud case against Swiss bank UBS AG and ordered him to start serving his 40-month term as scheduled this Friday.

The ruling by Fort Lauderdale Federal Judge William Zloch ignored both public and legal pleas for leniency by former UBS banker Bradley Birkenfeld, the main whistleblower in the high-profile case that cracked open Swiss banking secrecy.

Lawyers for Birkenfeld had asked Zloch last month to postpone the Jan. 8 start of his prison term, saying he was ready to cooperate further with the authorities in their pursuit of U.S. tax cheats.

The lawyers also requested reconsideration by Zloch of the jail term he gave Birkenfeld, a 44-year-old U.S. citizen who has been hailed by whistleblower advocates and U.S. prosecutors alike as a vital information source in the U.S. government’s case against UBS, his former employer.

The Swiss bank was targeted in a wide federal probe of thousands of U.S. tax cheats who hid assets in secretive UBS accounts.