The veil of secrecy that has protected the wealthy clients of Swiss banks for centuries was under severe threat last night after the US Justice Department moved to force UBS to reveal the names of 20,000 clients.

The US Government believes that the Swiss bank has helped some of the wealthiest Americans to evade $300 million (£152 million) in taxes.

Under Swiss law, UBS can reveal clients’ identities to a foreign regulator or law enforcer only if ordered to do so by the Swiss Government. However, the US Justice Department is discussing a settlement with UBS under which the department would make a request to the Swiss authorities for the names of the bank’s American clients. UBS declined to comment on any aspect of the Justice Department’s investigations except to say that it was treating them “with the utmost seriousness”.

The Justice Department, which has the power to bring criminal cases, has charged a former UBS banker and a consultant based in Liechtenstein – like Switzerland, a leading tax haven – with helping clients to avoid taxes by opening secret accounts.

It is working to broaden the investigation into UBS by examining its American account holders, whom it believes the Swiss bank may have helped to dodge at least $300 million in taxes by using undisclosed offshore accounts.

Even if UBS does not reveal the identities of its American account holders, many of its bankers and richest clients are in danger of being identified as part of a plea bargain between one of the group’s former bankers and the Justice Department.

If Bradley Birkenfeld, a former UBS banker, is called to testify he is expected to say that he helped Igor Olenicoff, a property developer in California, and other clients to evade taxes. Mr Olenicoff has pleaded guilty to filing a false tax return for 2002. Mr Birkenfeld, who helped him to hide $200 million, is hoping to agree a plea bargain to reduce his penalty. “He’s going to sing like a parakeet,” one of Mr Birkenfeld’s clients apparently said. Danny Onorato, Mr Birkenfeld’s lawyer, and the Justice Department both declined to comment.

Any disclosure would be a second big setback to UBS’s reputation as a conservative and watertight guardian of its customers’ money. The bank has been the biggest European victim in the US sub-prime mortgage crisis, with $38 billion in losses so far.

UBS’s shares have fallen by half in New York this year, cutting its market capitalisation to $61 billion. The bank still has $45 billion in US mortgage-related assets, $8.6 billion in leveraged loan commitments and $10.4 billion of American student loans on its books.

Separately, it emerged that Lehman Brothers has stepped up its quest for new capital to bolster its balance sheet after an estimated $4 billion of sub-prime-related losses this year. The US investment bank is understood to be considering releasing second-quarter results next week, a week earlier than scheduled, and coupling the disclosure with an announcement that it will raise up to $5 billion of fresh capital as it seeks to calm nervous investors.

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