MINERSVILLE, Pa. (AP) -- A former banker who provided key assistance in the U.S. tax evasion probe of Swiss banking giant UBS AG reported to prison Friday and said his cooperation should have earned him the federal government's gratitude, not time behind bars.

Bradley Birkenfeld, 44, a former private banker for UBS, said his whistle-blowing exposed "the largest tax fraud in the world" and allowed the Internal Revenue Service to recover billions of dollars in lost revenue.

Birkenfeld, who handled wealthy American clients of UBS from 2001 through 2006, pleaded guilty in June 2008 to a single count of conspiracy to defraud the U.S. and was sentenced to more than three years in prison.

"I think it's an injustice," he said Friday as he reported to a federal prison to begin his sentence. "I'm a proud American who did the best I could for my country and this is how they reward me."

Birkenfeld admitted helping clients hide hundreds of millions of dollars and evade U.S. taxes. His sentence has drawn criticism from whistle-blower advocates as too harsh because of Birkenfeld's importance in exposing tax evasion at UBS.

The Swiss bank last year paid a $780 million fine to the U.S. and agreed to turn over the names of 4,450 suspected tax dodgers.

Birkenfeld began providing inside information to U.S. authorities in 2007, describing how UBS helped rich Americans hide income from the IRS.

Prosecutors have said Birkenfeld did not initially reveal his own misconduct. He denies that. Birkenfeld filed a complaint earlier this week against prosecutors, accusing them of making false statements to the federal judge in Fort Lauderdale, Fla., who sentenced him to prison.