Switzerland's role as the world's private banker is under renewed attack as the Swiss authorities prepare to hand over confidential data on wealthy American clients of UBS to the US Department of Justice.

Some data, which relate to a US investigation into offshore services provided by UBS advisers to US clients between 2000 and 2007, have already been handed to US officials by UBS, according to a report in The New York Times.

UBS is accused of helping US clients avoid taxes by hiding assets in Switzerland. The Swiss bank has been trying to strike a settlement with the US over alleged breaches in time for its shareholders' meeting on October 2.

The bank refused to comment on the reports, except to say: "In these ongoing investigations UBS is working diligently with both Swiss and US government authorities, consistent with Swiss law and the legal frameworks for intergovernmental co-operation and assistance in an effort to achieve a satisfactory resolution of these matters."

Swiss law prohibits disclosure of client data or names, unless the country’s authorities believe the client has committed a serious crime, such as money laundering or tax fraud. Switzerland does not consider tax evasion to be a crime.

The Swiss Government has warned UBS not to breach Swiss banking secrecy rules by handing over client details to US authorities.

Any details would have to be given by UBS to the Swiss Government, which would then assess whether a serious crime has been committed before deciding whether to pass them on to the US authorities.

UBS is anxious to strike a deal with the US over its inquiries to give the business a fresh start under Peter Kurer, its new chairman.

However, hopes of an autumn deal appear increasingly unlikely as the US continues to demand data on the 19,000 US offshore clients.

Hans-Rudolf Merz, Switzerland's Finance Minister, recently warned UBS not to breach secrecy rules by providing details. "There is always the danger that one wants to quickly sort out the difficulties and somehow find a fast solution. Banking secrecy must be defended. Whoever undermines it would be acting illegally.

"UBS cannot just give out customer lists. That would infringe banking secrecy and I cannot accept that," he said.

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