BERN, Switzerland - Federal authorities have filed a lawsuit against Swiss-based bank UBS AG seeking the identities of tens of thousands of U.S. customers.

Two Swiss flags fly above a logo of the UBS on the top of the Swiss banking giant headquarters in Zurich. Swiss bankers are going back to the basics of cosy and discreet relationships with their clients after disastrous forays into the brash and boastful world of American investment banking, analysts say. (AFP/File/Fabrice Coffrini) The suit filed in Miami Thursday seeks to force the firm to turn over information on as many as 52,000 U.S. customers who hid their accounts from the U.S. government in violation of tax laws. According to the government's lawsuit, the accounts in question held about $14.8 billion in assets in the past decade.

The company said it will fight in court to keep the names private, arguing Swiss bank secrecy laws shield those customers.

A federal judge will now decide whether the U.S. courts can force a bank to violate Swiss bank secrecy laws and provide the account information.

The move came as Switzerland desperately sought to reassure its citizens and international banking clients that it would safeguard a treasured tradition of confidential accounts after taking the unprecedented step of revealing over 250 tax cheats to U.S. authorities.

"Banking secrecy, ladies and gentlemen, remains intact," President Hans-Rudolf Merz told reporters.

Unconvincing claim
His claim, however, failed to convince many critics of a deal struck between Swiss bank UBS AG and the U.S. Department of Justice - which is investigating allegations the bank helped thousands of wealthy American evade taxes.

Merz said Swiss authorities handed over the files on 250 to 300 American clients of UBS who are suspected of committing tax fraud. The transfer took place in the middle of the night in the Swiss capital Bern, just ahead of a U.S. deadline for Swiss cooperation, he said.

Experts said the decision to bypass the courts and give up customers before exhausting all legal options seriously endangers a pillar of the banking industry that helped transform Switzerland into one of the world's richest countries.

Rainer Schweizer, professor of public law at the University of St. Gallen, said the foundations of Switzerland's legal system have now been shaken.

Undeclared money was no longer safe in Switzerland, said Susan Emmenegger, professor of banking at the University of Bern.

Zurich lawyers immediately filed a suit against the head of Switzerland's financial services authority FINMA, which authorized the transfer of files.

"This so-called agreement is a brutal demonstration for why banking secrecy should be guaranteed in the Swiss constitution," said Hans Geiger, emeritus professor of banking at the University of Zurich, one of many leading Swiss experts on the industry to voice his opposition Thursday.

Switzerland's largest bank and U.S. officials have been negotiating intensely since allegations surfaced last year that UBS helped wealthy Americans conceal up to $20 billion.

Merz, UBS and Switzerland's financial regulator all insisted that Thursday's handover was not a retreat from the principle of banking secrecy because it concerns only a small number of files that are linked to tax fraud - and not tax evasion.

Pillar of Swiss identity
Along with neutrality, banking secrecy is a pillar of Swiss national identity. Under a 75-year-old law, banking secrecy can only be lifted when individuals are deemed to have deliberately defrauded tax authorities as opposed to failing to declare all assets, a distinction only Switzerland and other tax havens make.

Banking secrecy "serves to protect privacy," Merz said. "However, it does not protect tax fraudsters."

Under Wednesday's deal, UBS also agreed to pay a $780 million fine and cooperate further with U.S. authorities.

The Swiss Bankers Association said it regretted the settlement because it was agreed before a legal process initiated last year between U.S. and Swiss authorities had been completed.

The decision to act quickly, rather than wait for Switzerland's often slow and costly legal system to approve the transfer of the files, follows intense pressure from Washington.

After indicting a former UBS official in May, American prosecutors stepped up their probe of UBS last November by indicting senior UBS banker Raoul Weil and warning that other top executives could face similar charges. One name mentioned was Peter Kurer, the bank's former general counsel and current chairman.

Congress is expected to discuss UBS in a hearing on tax havens Tuesday.

Merz said U.S. indictments of the bank's most senior staff, along with a wider investigation into its business practices in the United States, would have threatened the Swiss economy as a whole during a serious economic downturn.

The bank's bankruptcy would cost the Swiss economy up to $250 billion, Merz said.

UBS lost 19.7 billion Swiss francs last year, the biggest corporate loss in the nation's history, and received nearly $60 billion in a state bailout in October.

The Associated Press, Reuters and msnbc.com staff contributed to this report.