US prosecutors have asked a federal judge to issue a summons requiring UBS to turn over information about US taxpayers who may be using Swiss bank accounts to evade federal income taxes.

The move comes a little over a week after Bradley Birkenfeld, a former UBS private banker, pleaded guilty to charges that he conspired to help a US billionaire evade income taxes.

The news came on a day when UBS shares fell to their lowest point in about 10 years on Monday on fears that Europe’s biggest casualty of the US subprime crisis might on Tuesday announce further writedowns.

The Justice Department said on Monday that it had filed court papers seeking permission to allow the Internal Revenue Service to issue a so-called “John Doe” summons on UBS. The IRS uses such a summons to obtain information about possible tax fraud by people whose identities are unknown. If the request for the summons is approved, it would direct UBS to produce records identifying US taxpayers with accounts at the bank who elected to have their accounts hidden from the IRS, prosecutors said.

In his statement to the court earlier this month, Mr Birkenfeld said he and other UBS bankers had assisted US clients in concealing their ownership of the assets held offshore. He said he took these actions “to prevent the risk of losing the about $20bn of assets under management in the US …which earned the bank approximately $200m per year in revenues”.

John DiCicco, deputy assistant attorney-general for the Justice Department’s tax division, said: “We are working co-operatively with both the Swiss government and UBS to obtain this information. We are prepared to seek enforcement if that process is not successful.”

UBS said on Monday it was aware of the request and added: “As stated, UBS takes this seriously and is working diligently with both Swiss and US government authorities.” UBS, which has plunged into loss on the back of almost $38bn in writedowns, declined to comment on fears of a new profits warning after the end of the second-quarter trading period.

Analysts have estimated the bank could face between $2.5bn and $7.5bn in new losses. The latest estimates, which come in spite of efforts by the Swiss bank to reduce its subprime portfolio, follow cuts in the credit ratings of monoline insurers to which UBS remains exposed.

The bank’s second-quarter results are not due until August 12. UBS shares closed down 4.29 per cent at SFr21.44 on Monday.

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