A court says the handing over of confidential UBS bank details to United States investigators by the Swiss authorities last year was illegal.
The federal administrative court said the Swiss Financial and Market Supervisory Authority (Finma) abused its power when it ordered that details of 285 account holders suspected of tax evasion in the US be sent to Washington.
“Finma could not base its decision on emergency rights enshrined in the constitution,” a statement said on Friday.
The government did not give the green light to apply the emergency clause, but only asked Finma to take the necessary steps to prevent a legal complaint by the US against UBS, the statement added.
Observers say the regulatory authorities are likely to lodge an appeal against the ruling.
But the ruling is seen as a setback for Finance Minister Hans-Rudolf Merz.
For its part, Finma on Friday reiterated that its decision was based on consultations with the government and aimed at protecting the bank and the Swiss economy from serious damage.
Last February Finma and UBS announced they would disclose details of 255 clients suspected of tax evasion by US authorities.
UBS paid $780 million (SFr806.5 million) in fines last February after admitting "improper activities" from some of its employees.
Following intense pressure, the Swiss government in August also agreed to hand over a further 4,450 account holder details to the US Internal Revenue Service investigating cases of suspected tax fraud.
As agreed with the US government as part of a deal the first 600 files were processed by the end of last year and will be handed over pending court appeals.
Urs Geiser, swissinfo.ch