Switzerland has signed a tentative agreement to share information with the United States about the billions of dollars that US citizens have stashed in secret Swiss bank accounts.

However, the agreement could fall apart if America’s Inland Revenue Service (IRS) continues to sue UBS, Switzerland’s biggest bank, for the names of 52,000 American customers with offshore bank accounts.

Switzerland wants to cut bilateral deals with 12 countries by the end of this year to get off a list of countries drawn up by the Organisation for Economic Co-operation and Development that should improve their tax co-operation. So far Switzerland, whose private banks hold an estimated $2 trillion for wealthy people worldwide, has reached agreements with Denmark, Norway, France, Mexico and another unnamed country.

A spokesman for the Swiss Finance Ministry said that the country had reached an agreement with the US on Thursday, but that the Swiss parliament may refuse to ratify the deal if the IRS continues its civil case against UBS.

The IRS, the division of the US Treasury that handles taxation, launched a lawsuit in February to force UBS to hand over the names of 52,000 Americans with offshore accounts at the bank. In a response filed with a court in Miami in May, UBS said that giving up the names would cause the bank to break strict Swiss banking secrecy laws. Any information exchange should be government-to-government, the bank said.

The IRS is to file its response to UBS’s filing on June 30. The case will open on July 13.

UBS declined to comment on Switzerland’s new agreement with the United States. Shares in the bank were flat at lunchtime in New York at $13.28, as investors considered the implications of the agreement.

President Merz of Switzerland had previously told Timothy Geithner, the US Treasury Secretary, that his country was keen to go ahead with a bilateral tax agreement if the IRS dropped its case against UBS. Mr Geithner did not mention the case yesterday when he welcomed the new agreement. “This treaty will increase our ability to enforce our tax laws and will help bring an end to the era of offshore accounts and investments being used for tax evasion,” he said.

President Obama has made the eradication of tax evasion through offshore accounts one of his main concerns. Under US law, taxpayers with more than $10,000 (£6,000) in offshore accounts must disclose the sums to the tax authorities — but the IRS cannot catch individuals disobeying the law without knowing who they are and where their accounts are held.

In February, UBS avoided criminal prosecution in America for helping rich Americans to avoid paying tax on assets held overseas. The bank paid $780 million in fines and disbursements and gave the IRS the names of more than 250 customers. Two of these clients have since been prosecuted. A day after UBS agreed to pay the penalty, the IRS bought its civil suit seeking many more names.

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