UBS is the largest bank in a famously secretive system
Switzerland's largest bank, UBS, has asked a US court not to go ahead with a tax case involving more than 50,000 US customers with Swiss accounts.
UBS told a federal court in Florida it would violate Swiss laws on banking secrecy if it provided the information on its clients.
The US suspects 52,000 Americans of using UBS accounts to hide almost $15bn of assets and unpaid taxes.
Switzerland only recently signed up to global rules on bank data sharing.
It decided in March to ease banking secrecy and fully adopt accepted tax standards. The government agreed to begin negotiations with the US and Japan on tax co-operation.
Correspondents say the US case involving UBS is a sign it is stepping up its campaign against tax evasion - and directly challenging the tradition of Swiss banking secrecy.
The Internal Revenue Service, which administers tax in the US, has taken out a civil suit to force UBS to reveal the identities of 52,000 American customers suspected of holding accounts totalling $14.8bn.
However, the bank has now told the court that it cannot hand over the information without violating Swiss law.
UBS says no specific evidence has been presented against its clients, meaning it is unable to waive bank secrecy rules.
"Switzerland's laws prohibit the release of confidential information to foreign governments when the request has not been made through authorised inter-governmental channels," the country's government said.
"If the court were to order UBS to produce evidence from Switzerland, and backed that order with coercive powers, the court would be substituting its own authority for that of the competent Swiss authorities, and therefore would violate Swiss sovereignty and international law," it added.
Earlier this year, UBS did cave in to US demands in a separate case involving about 300 customers.
The bank agreed to pay more than $700m in an out of court settlement.
US and Swiss officials have begun negotiations on a new tax treaty that Washington hopes will help it track tax evaders.
Swiss officials, who are also under pressure from the European Union, say it could take until the end of the year to reach an agreement.