FORT LAUDERDALE, Fla. — A former private banker for the Swiss giant UBS pleaded guilty in Federal District Court on Thursday to a charge of helping a wealthy American real estate developer evade taxes on $200 million.

The former banker, Bradley C. Birkenfeld, in his plea, also agreed to cooperate with prosecutors in a widening investigation of the bank’s practices and its clients, a decision that could eventually help shine a light on Swiss banking secrets and reveal the names of scores of wealthy American clients.

Mr. Birkenfeld, 43, was accused of helping the developer, who has been identified as Igor Olenicoff, hide more than $200 million in accounts in Switzerland and Liechtenstein. But Mr. Birkenfeld had “numerous” other American clients, according to his lawyer, Danny Onorato.

Mr. Birkenfeld told Judge William J. Zloch that he knew he was breaking the law but did so because of “incentives” — meaning large bonuses paid on top of his salary.

In a seven-page statement of facts, Mr. Birkenfeld described some of the tactics of the private banking trade. On one occasion, according to the court document, Mr. Birkenfeld, at the request of an unidentified American client, bought diamonds using the client’s Swiss bank account and then smuggled them into the United States in a toothpaste tube. UBS trained its private bankers traveling to the United States to tell customs authorities that the trip was for pleasure, not business, according to the court papers.

The investigation of UBS’s private banking practices is the first in this country to attempt to peel back the layers of secrecy surrounding Swiss banks, whose tradition dates to the Middle Ages. Prosecutors and lawyers for Mr. Birkenfeld contend that he was not a rogue employee but rather part of a widespread effort by the bank to flout United States tax laws with its offshore banking services sold to wealthy Americans.

“Our position is that Mr. Birkenfeld was working at UBS where people knew exactly what was going on,” Mr. Onorato said after the hearing.

“He’s going to tell the government everything he knows. He is going to cooperate with the government fully,” Mr. Onorato said, adding that Mr. Birkenfeld “had numerous U.S. clients.”

Mr. Birkenfeld, a director of major wealthy American clients with offshore accounts for UBS in Geneva from 2002 to 2006, was indicted in May along with a Liechtenstein finance executive, Mario Staggl.

Both men were accused of creating bogus trusts and sham offshore entities to hide some $20 billion in offshore assets owned by wealthy American clients. UBS is in talks with the Justice Department and Swiss authorities about turning over the names of up to 20,000 American clients.

Mr. Birkenfeld’s work concerned offshore undeclared assets held for wealthy Americans. In his statement of facts, he said that he and other unidentified UBS private bankers urged their American clients to destroy all offshore private banking records held in the United States; to use Swiss credit cards that could not be discovered by American tax authorities; and to falsely characterize money pulled out of Swiss accounts as loans from UBS. The entire business brought in $200 million a year in revenues to UBS.

Prosecutors have been shifting their focus on tax evasion to private banking practices that use undeclared offshore accounts in European tax havens.

“Mr. Birkenfeld’s cooperation is anticipated to assist the government nationwide with respect to its investigation,” Kevin Downing, a senior trial lawyer for the Justice Department, told the court. “We have an ongoing investigation.”

Mr. Birkenfeld will be sentenced in August.