FORTUNE -- Are you among the suspected tens of thousands of Americans with a secret Swiss bank account that you are still hiding from the Internal Revenue Service? If so, you are about to acquire a Matterhorn-sized headache.
Switzerland made a desperate decision on Wednesday to save its battered private banking sector by allowing Swiss banks to cooperate with a broad tax evasion probe by U.S. investigators. It is likely to unleash a flood of fresh disclosures to the IRS by American taxpayers, lawyers say.
Such disclosures could trigger hefty fines that can reach a multiple of what an account holds. To stand a chance of reducing those fines, Americans with hidden Swiss accounts "should run, not walk, to jump in line with an IRS voluntary disclosure in light of this move by the Swiss government," says Josh Ungerman, a tax and estates lawyer in Dallas and a former IRS prosecutor.
Lawrence Horn, a tax and business crimes lawyer in Newark, N.J. and a former federal prosecutor, says he expects "at least 10,000" American taxpayers to come forward in the next 12 months or so.
The Swiss decision has two key points: Over the course of 12 months, banks will be allowed to turn over to American authorities general statistical data on their work with American clients. More significantly, the U.S. can seek concrete account details -- with names of taxpayers -- if the U.S. Senate gets its act together and passes a double-taxation treaty already green-lighted by Bern.
