Switzerland is now examining whether it can assist in the request, the spokesman, Folco Galli, was quoted as saying, but he declined to give further details on the request, which arrived last Wednesday.

Bradley Birkenfeld, a former UBS executive, has been charged with conspiring to defraud the  US by helping wealthy clients hide assets and evade taxes.

The case against Mr Birkenfeld led the US Justice Department to investigate whether the bank itself had helped US clients evade taxes from 2000 to 2007.

Mr Birkenfeld, a key clients director for UBS in Geneva from 2002 to 2006, is a partner of Union Charter Financial, a Geneva firm that caters to wealthy investors. He was indicted along with Mario Staggl, a trust manager from Liechtenstein. Both are accused of creating fictitious trusts and bogus corporations to conceal the ownership of offshore assets, and of advising clients to destroy bank records.

Swiss law prevents banks from divulging the names and details of their clients except in cases of tax fraud. Tax evasion is not considered sufficient grounds for legal assistance.

The US investigation has already affected the Swiss bank's operations in the country, where it manages about $704 billion for rich American clients.

Last November UBS told some private banking clients they would in the future have to travel to Switzerland if they wanted to speak to their advisors.

The Swiss media have reported that UBS was concerned its employees could face arrest if they travel to the  US.

The bank has declined to comment on the reports.

Mr Birkenfeld is to appear before a court in Fort Lauderdale, Florida, on Thursday.