By Tom Brown

FORT LAUDERDALE, Florida (Reuters) - A former UBS AG (UBSN.VX: Quote, Profile, Research) banker pleaded guilty in federal court on Thursday to helping an unidentified billionaire real estate developer hide $200 million (101 million pounds) in assets from U.S. tax authorities.

Bradley Birkenfeld, a 43-year-old U.S. citizen, told a court in Fort Lauderdale that he was a UBS banker at the time, received a large salary, and was "incentivized" by his employer to carry out the activities that led to the charges.

He had previously pleaded not guilty. His change of heart and subsequent plea deal could affect an ongoing U.S. investigation of UBS's conduct in relation to services it provided to U.S. clients from 2000 through 2007, and crack open Switzerland's much-vaunted tradition of banking secrecy.

"He's going to tell the government everything he knows about what was going on at UBS," Birkenfeld's lead attorney, Danny Onorato, told reporters after his court appearance.

U.S. media have reported that UBS is considering whether to reveal the names of up to 20,000 wealthy American clients as federal authorities intensify the probe into offshore bank accounts.

In evidence he gave the court, Birkenfeld said he and other UBS bankers helped the bank earn $200 million a year managing around $20 billion in assets held in offshore tax havens and concealed from the U.S. tax authorities by wealthy Americans, the U.S. Attorney's Office for the Southern District of Florida said.

"Mr. Birkenfeld had numerous U.S. clients. A number of these individuals have been reaching out to U.S. authorities as a result of Mr. Birkenfeld's case," Onorato said.

Unsealed last month, the grand jury indictment against Birkenfeld charged him with conspiring to defraud the United States by helping the unidentified U.S. billionaire create bogus corporations and other entities to conceal the ownership of hidden offshore assets.