American clients who hid more than SFr1 million ($980,000) in undeclared bank accounts with Swiss bank UBS could have their details handed over to the United States.
The Swiss justice ministry on Tuesday unveiled the specifics surrounding which 4,450 UBS customers risk having their financial data disclosed to the US Internal Revenue Service (IRS).
The agreement is part of a deal struck between US and Swiss authorities in August to end a major tax-evasion investigation against UBS.
Potentially affected clients include Americans living in the US or elsewhere who controlled offshore company accounts from 2001 onwards.
Also targeted are clients with at least SFr250,000 who are suspected of "fraudulent behaviour" such as by falsifying documents. Customers who earned an average of SFr100,000 a year or more for three consecutive years could also come under scrutiny.
The Swiss authorities have until the end of August 2010 to hand over the names to their US counterparts.
The vast majority of cases involves matters of tax evasion, Michael Leupold of the Federal Justice Office said at a news conference on Tuesday.
Senior finance ministry officials said the procedure was on course and respected Swiss law.
Last August the Swiss government agreed to give account details of up to 4,450 Americans suspected of serious tax evasion after UBS acknowledged it had helped thousands of American clients hide assets from the IRS.
The move is widely seen as a break with Switzerland's traditional banking secrecy for foreigners.
Meanwhile, more than 14,700 US taxpayers have come forward to disclose billions in offshore bank accounts in 70 countries, including Switzerland, under a voluntary programme, according to IRS officials.
An amnesty programme for tax dodgers expired on October 15.
Last week the US ambassador to Bern said about 9,000 Americans with offshore accounts in Switzerland had taken advantage of the amesty offer.
swissinfo.ch with agencies