FORT LAUDERDALE, Fla. — One after another, the secrets spilled out. The millions stashed in Swiss bank accounts. The precious jewels and artwork whisked into hiding. The diamonds smuggled in a tube of toothpaste.

Bradley C. Birkenfeld kept all those secrets and more as he discreetly managed the fortunes of American millionaires and billionaires at UBS, the Swiss bank.

But on Thursday, Mr. Birkenfeld broke his silence, and with it the omerta of Swiss banking, in Federal District Court here. He pleaded guilty to a charge of helping an American billionaire, Igor Olenicoff, evade millions of dollars in federal income taxes.

Mr. Birkenfeld’s testimony, the centerpiece of a widening investigation into UBS and its wealthy American clients, blew a hole in the wall of secrecy surrounding the world of Swiss banking. Under pressure from the authorities, UBS has been considering disclosing the names of thousands of its wealthy clients in the United States.

In a seven-page statement to the court, Mr. Birkenfeld, a 43-year-old American, said UBS bankers used a variety of ruses to court American clients and help them dodge taxes. The document reads like a how-to of high-end tax evasion.

UBS advised bankers traveling to the United States to tell airport customs officials that the visit was for pleasure, not business, Mr. Birkenfeld said. The bank urged clients to destroy banking records to conceal their offshore accounts.

Some American clients were told to stash watches, jewelry and artwork that they had bought with money hidden offshore in safe deposit boxes in Switzerland. UBS also encouraged them to use Swiss credit cards so the Internal Revenue Service could not track their purchases, Mr. Birkenfeld said.

The list went on. Mr. Birkenfeld said that at times he served as a personal courier for his clients, spiriting checks out of the United States and depositing them in accounts in Denmark, Switzerland and Liechtenstein, away from the prying eyes of the I.R.S. At one point, he wrote, he even smuggled diamonds for a customer in a tube of toothpaste.

Mr. Birkenfeld was a Geneva-based director of wealthy American clients with undeclared offshore accounts with UBS from 2002 to 2006. Over all, that business oversaw $20 billion in assets and brought in revenue of $200 million a year, according to court papers.

UBS declined to comment directly on Mr. Birkenfeld’s testimony. In a brief statement, the bank said it was cooperating with Swiss and United States authorities.

In court, Mr. Birkenfeld wore a charcoal pinstripe suit that was as solemn as his face. He told Judge William J. Zloch that he knew he was breaking the law but did so because of the “incentives” UBS offered him, meaning large bonuses.

Judge Zloch asked if he had not been troubled by his actions.

“It did concern me, your honor,” Mr. Birkenfeld said.

Mr. Birkenfeld pleaded guilty to helping Mr. Olenicoff evade $7.2 million in taxes on $200 million in hidden offshore assets. Mr. Birkenfeld will be sentenced in August. Mr. Olenicoff pleaded guilty last year to filing a false 2002 tax return.

The unfolding case is an embarrassment for UBS, the world’s largest money manager for the wealthy. Mr. Birkenfeld’s cooperation is vital to the inquiry.

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

The Swiss government and private sector have sent a delegation to Washington to meet with Justice Department officials Friday, according to a person briefed on the matter. The trip follows a request by the Justice Department to the Swiss government last week for assistance in the investigation of UBS. Among other things, the Justice Department is seeking to force UBS to turn over the names of up to 20,000 American offshore clients who may have violated United States tax laws.

Unlike in the United States, tax evasion in Switzerland is not illegal. Federal prosecutors contend that they, not Switzerland, have jurisdiction over any violations of United States tax laws on American soil.

The UBS inquiry is causing jitters among wealthy American clients, some of whom have been contacting the American authorities, according to Danny Onorato, a lawyer for Mr. Birkenfeld. Prosecutors and lawyers for Mr. Birkenfeld contend that he was not a rogue employee but rather part of an broad effort by the bank to flout American tax laws with 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,” he said, adding that Mr. Birkenfeld “had numerous U.S. clients.”

Mr. Birkenfeld’s statement said he had provided evidence that “managers and bankers at the firm, including Birkenfeld,” assisted American clients in concealing their ownership of assets held offshore.

Mr. Birkenfeld’s former boss, Martin Liechti, a Swiss national who is one of the world’s top private bankers, was detained recently by United States authorities but has not been charged, though he is not allowed to leave the country.

Mr. Birkenfeld’s indictment was unsealed in May along with that of a Liechtenstein finance executive, Mario Staggl. Both men were accused of creating bogus trusts and sham offshore entities to hide offshore assets owned by wealthy Americans.

Times Topics: UBS A.G.

* Comment in brackets added by John Donovan