UBS, the embattled Swiss banking group, was under fresh pressure yesterday after it emerged that one of its senior executives had been detained as part of an American tax investigation.

Martin Liechti, a senior banker, was held by authorities in the United States investigating whether the Swiss bank had helped its American clients to avoid taxes.

Mr Liechti, the Zurich-based head of North and South America for UBS's international wealth management business, is believed to have been detained as a material witness while visiting Miami last month as part of the investigation by the US Justice Department into advice given by UBS to clients over a period of several years ending in 2007.

A spokesman for UBS refused to confirm or deny that Mr Liechti was the banker involved, but he did confirm that one of its senior bankers had been detained. The bank said that the banker had not been charged and that UBS was co-operating with the inquiry.

UBS said: “Our understanding is that the respective employee, who has not been charged with any wrongdoing by the US Government, will remain in the United States pending discussions with the US authorities regarding resolution of his status as a witness.”

There were unconfirmed reports that a former UBS employee had been providing investigators with key information about the bank's tax advice business and clients in the US.

In November, UBS closed down its Swiss-based US banking operation with the loss of 70 jobs as part of a wider reshuffle of its global wealth management division.

Last June, UBS published Country Paper USA, an eight-page document that attempted to tighten up compliance for private wealth managers travelling to America. The paper, nearly three times larger than the previous document advising UBS's managers travelling abroad, stated: “Failure to comply with US rules and regulations may result in substantial civil, administrative and criminal sanctions against UBS and its employees and could have a substantial negative impact on UBS's reputation in the US.”

The incident represents an unexpected fresh blow for the Swiss bank, which only yesterday announced a first-quarter net loss of SwFr11.5billion (£5.5 billion) and 5,500 layoffs as it struggles to deal with the impact of the credit crisis.

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