The Swiss authorities have already passed over 500 accounts of UBS clients to the US under last August’s agreement to resolve a tax dispute, according to Sonntag, the Swiss newspaper.

The information about the data transfer came after the Swiss lower house rejected the US-Swiss data transfer deal last Tuesday. The vote has led to a new parliamentary debate and has held up a final decision on whether to allow the government to hand over a total of 4,450 UBS client accounts to US authorities.

The US-Swiss deal, agreed last August, was made to resolve a civil case. In a separate, criminal case in February last year, the Swiss bank agreed to pay a $780 million fine to settle charges that it helped US citizens evade taxes. As a result of this case, UBS no longer provides offshore banking services to US clients. A number of other Swiss banks, such as Julius Baer and Wegelin, have also moved to stop providing such services.

UBS has already handed the accounts to Swiss tax authorities but the Swiss political dispute could prevent Switzerland handing them on to the US by the August deadline, according to Reuters, which carried a translation of the Swiss newspaper report.

Around 2,900 files had already been prepared for handover and 1,550 more accounts were still being processed, the newspaper said.

"Client dossiers were transferred to the United States in around 500 of the 2,900 cases," the paper cited the Swiss tax office spokesman Thomas Brueckner as saying. This transfer was possible because these 500 clients agreed to their data being send to the US.