MIAMI (Reuters) - The key informant in the U.S. tax fraud case against Swiss bank UBS AG says he does not deserve the federal prison term he is due to start serving next month, according to an interview to be broadcast on Sunday.

Former UBS banker Bradley Birkenfeld runs into the Federal Courthouse in Ft Lauderdale, Florida, August 21, 2009. REUTERS/Andrew Innerarity

Bradley Birkenfeld, a 44-year-old U.S. citizen, has been hailed by whistleblower advocates and U.S. prosecutors alike as pivotal to the case against UBS, his former employer. The bank was targeted in a wide probe by U.S. authorities for helping U.S. tax cheats to hide assets in UBS accounts.

Speaking in an interview to be broadcast on CBS television’s “60 Minutes” on Sunday, his first since he began providing insider information to U.S. prosecutors in the summer of 2007, Birkenfeld seems both angered and stunned by the fact that he is due to enter prison on January 8 for a 40-month term.

Birkenfeld was handed his sentence by a Florida district court judge in August, two days after U.S. and Swiss authorities signed a pact in which Switzerland agreed to reveal the names of about 4,450 wealthy American clients of UBS to U.S. tax investigators.

No other UBS bankers have been jailed in connection with the massive tax fraud case. Birkenfeld’s former boss, a Swiss citizen and an alleged mastermind of the conspiracy, was detained in the United States in 2008 and held for four months on a material witness warrant before he was quietly allowed to leave the country.

“I gave them the biggest tax fraud case in the world,” Birkenfeld says in the interview, portions of which were released by CBS on Wednesday.

“I exposed 19,000 international criminals and I’m going to jail for that?” he asks.

Birkenfeld pleaded guilty to a single fraud conspiracy count in June 2008, when he acknowledged helping his largest U.S. client hide assets from the Internal Revenue Service.

In a claim disputed by Birkenfeld’s lawyers in a December 7 letter to U.S. Attorney General Eric Holder, Justice Department officials say the jail time was justified because he was not initially forthcoming about the tax fraud committed by his billionaire U.S. client Igor Olenicoff.

In the “60 Minutes” interview, Birkenfeld, who says he was sometimes asked to “cater” to his U.S. clients by shopping for things like cars, chalets or expensive Swiss watches on their behalf, was reminded by journalist Steve Kroft that he was an enabler for people breaking the law.

“And I am the only one going to prison. Out of 19,000 accounts and no Swiss bankers,” Birkenfeld responds.

In a December 26 filing with the U.S. District Court in Florida where he was sentenced, Birkenfeld’s attorneys asked that his prison time be postponed and that a hearing be scheduled for the judge to reconsider the sentence.

There has been no immediate comment from prosecutors but a government response was expected to be filed with the court by Monday, a person familiar with the situation told Reuters.

Earlier this year, the bank agreed to pay $780 million and exit the U.S. tax-shelter business to settle criminal charges in the tax evasion case, which ended up cracking open Switzerland’s jealously preserved banking secrecy.