Swiss authorities said on Thursday that they had handed over to US authorities data on about half of the 4,450 American clients of Swiss bank UBS who were suspected of tax evasion by an August 26 deadline.
However, the federal finance department indicated that the United States and Switzerland had agreed to prolong the handover into the autumn under a deal to avert Washington's threatened lawsuit against one of Switzerland's biggest banks.
The handover, which marks a softening of Switzerland's coveted banking secrecy, is the cornerstone of an accord the two governments struck in August 2009 on the tax evasion case.
"Within the scope of this administrative assistance, the Federal Tax Administration examined approximately 4,450 UBS client accounts and issued the corresponding conclusive decrees by the prescribed deadline," the finance department said in a statement.
However, data was only physically supplied to the United States "in around half of the cases."
Swiss and US officials were holding talks about "the final stage of the agreement's implementation.
"Both parties are optimistic that the US authorities will receive most of the agreed account information within a reasonable period of time and that the US authorities will definitively withdraw the civil action (John Doe Summons) brought against UBS," the statement added.
The US Internal Revenue Service warned in June it was ready to renew legal action if Switzerland failed to meet the terms of the August 2009 deal obliging UBS to provide the names of customers allegedly dodging American taxes.
US tax authorities started their offensive against UBS in 2008 after questioning a former banker, prosecuting the bank through US courts and forcing it to hand over 300 client names and pay a 780-million-dollar fine.
The IRS subsequently threatened the Swiss bank, which by then was struggling to survive the global financial crisis, with more litigation over another 52,000 suspected tax evaders.
The lawsuit was averted after the Swiss government intervened to negotiate a settlement with Washington, fearing permanent damage to Switzerland's then biggest bank that could have undermined its whole banking system.
The UBS case also sharpened broader international pressure on the country's tradition of banking secrecy, forcing Swiss authorities to narrow its scope in March.