Bradley Birkenfeld, who is pleading guilty to charges of conspiracy at a federal court in Ft Lauderdale, Florida, said that he himself had advised clients on a string of tax dodging schemes.
These included buying up artwork and expensive jewelry through the use of offshore accounts, putting money and jewels into Swiss safety deposit boxes and setting up bank accounts in other people's names.
On one occasion, the banker even agreed to buy diamonds for an American client - and to smuggle them out of the country in a toothpaste tube.
The Justice Department is aiming to make further arrests at UBS due to Mr Birkenfeld's evidence.
"Rather than risk losing the approximately $20 billion of assets under management in the United States undeclared business [UBS] assistedâ¦wealthy US clients in concealing their ownership of the assets held offshore," the banker said in a statement accompanying his guilty plea.