NEW YORK (Reuters) - A former UBS Group AG banker who as a whistleblower helped U.S. authorities prosecute the Swiss bank for tax fraud, only to spend 2-1/2 years in prison for helping a billionaire client evade taxes, on Monday filed a $20 million libel lawsuit against his former employer.

FILE PHOTO - Bradley Birkenfeld makes remarks before surrendering to authorities at the Schuylkill County Federal Correctional Institution in Minersville, Pennsylvania, January 8, 2010. REUTERS/Tim Shaffer

Bradley Birkenfeld, who in 2012 got a record $104 million award from an Internal Revenue Service whistleblower program, faulted UBS over statements published last November and this month by the New York Post and Bloomberg BNA Daily Tax Report.

Birkenfeld said UBS acted with actual malice by referring to his “often unsubstantiated” recollections in a recent book and having been “convicted in the U.S. for, among other things, having lied to the U.S. authorities.”

He said UBS did this as part of an international campaign to impede his effort to expose its “decades-long wrongdoing,” and undercut the credibility and sales of his book “Lucifer’s Banker: The Untold Story of How I Destroyed Swiss Bank Secrecy.”

UBS had no immediate comment on the lawsuit, which was filed in a New York state court in Manhattan and seeks $10 million of both compensatory and punitive damages.

Birkenfeld also named Peter Stack, UBS’ head of media relations in the Americas, as a defendant. The New York Post and Bloomberg are not defendants.

In an interview, Birkenfeld, the subject of a 2010 profile on CBS’ “60 Minutes,” said he sued “to hold UBS accountable.” His lawyer did not immediately respond to requests for comment.

Birkenfeld provided tips that led UBS in 2010 to pay a $780 million U.S. fine for helping about 19,000 wealthy Americans hide up to $20 billion in secret bank accounts.

More recently, Birkenfeld testified in a similar probe involving the bank in France.

He went to prison after pleading guilty in 2008 to a charge of conspiring to defraud the United States in connection with his client Igor Olenicoff, a real estate developer.

Olenicoff had pleaded guilty in 2007 to filing a false tax return but did not serve prison time.

Birkenfeld’s lawsuit noted that the Post clarified its article to show he was “never charged with or convicted of perjury or lying to U.S. investigatory authorities.”

Now 52, Birkenfeld said in the interview he now works with whistleblowers “so they can get their message out and eradicate waste, fraud and corruption in government, as well as corporations.”

The case is Birkenfeld v UBS AG et al, New York State Supreme Court, New York County.