UBS has admitted giving details to the United States of US onshore bank accounts as part of a tax probe that is testing Swiss bank secrecy and UBS's reputation.
UBS and the Swiss finance ministry deny however any data has been transferred from offshore or undeclared bank accounts located in Switzerland that are key to the country's long-standing tradition of bank confidentiality.
The probe focuses on whether UBS, Switzerland's largest bank, illegally helped wealthy Americans to dodge taxes through its offering of offshore services from 2000 to 2007.
The bank was singled out by US President-elect Barack Obama as one of those which helped "tax cheats".
The Swiss are still assessing a request by US authorities for information about offshore bank accounts held by American UBS clients in Switzerland. On Tuesday finance minister Hans-Rudolf Merz said he recognised the scale of the investigation.
Tax evasion is not a crime in Switzerland and the finance ministry is the only authority in the country that could authorise the transfer of Swiss-based client data to the United States.