* US clients allege Swiss bank failed to advise properly

* Lawsuits charge asset bankers have legal obligation to clients

The case of a wealthy U.S. businessman who pleaded guilty to evading taxes but then sued the Swiss bank where he hid his money is scheduled to go to trial on May 8, the first major test of civil legal challenges to Swiss banks that sold offshore private banking services to help Americans evade taxes.

The civil suit, filed against UBS AG in federal court in Santa Ana, California - and another filed against UBS in federal court in Chicago - will probe whether clients can legally rely on their private bankers' assertions there is no need to disclose the accounts on their tax returns or sign required disclosures.