An American client of UBS has gone to court to challenge Switzerland’s controversial plan to divulge details of once-secret bank accounts to US investigators.

The lawsuit, filed at the Swiss Federal Administrative Court yesterday, could upset the Swiss Government’s pledge to cooperate with a US investigation into whether UBS helped Americans to evade taxes by hiding money in Swiss accounts.

Switzerland, which is under intense pressure from the United States while investigators examine its largest bank, agreed to a controversial request to release some private data. That step has damaged its traditional selling point – banking secrecy.

If the client, who has not been identified, is successful in his appeal, lawyers expect a barrage of similar lawsuits from American clients who face criminal prosecution if the US finds information showing that they have evaded taxes. The appeal will be watched in Swiss banking circles because a win for the American customer may restore some confidence to a nervous industry. If the appeal is dismissed and the court approves the data transfer, lawyers said that there would be legal precedent for further moves by the US Government.

The six-month American investigation into UBS, which is being run by the Department of Justice, unveiled its first senior prosecution last week when a member of the bank’s group executive board was charged with conspiring to evade taxes.

Raoul Weil, 49, chairman and chief executive of global wealth management at UBS, is accused of helping 20,000 American customers to hide $20 billion of assets. Mr Weil, who has stepped down from his job, is believed to be in Switzerland, which does not typically extradite citizens to the US.

Separately, Ramesh Singh, the head of UBS’s asset-backed securities business, the group responsible for billions of dollars of losses in recent months, is leaving the bank. The unit run by Mr Singh, a 19-year veteran with the bank, contributed to UBS racking up the largest credit crunch-related losses of any European bank.

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