Shares in UBS plunged on Friday after U.S. tax authorities said they were widening a probe into the Swiss bank and wanted 52,000 client names, a day after UBS agreed to a hefty fine to settle criminal charges.

The stock was down 9.4 percent at 11.60 Swiss francs at 3:29 a.m. EST, reversing some of Thursday's strong gains. The DJ Stoxx index of European banks was down 3.8 percent, and rival Credit Suisse was down 8.65 percent.

Analysts had welcomed Wednesday's news in the hope it would settle the U.S. probe, which is investigating help given to rich Americans to dodge tax, and allow crisis-hit UBS to focus on a badly needed restructuring.

But the civil U.S. lawsuit announced late on Thursday will continue to keep UBS under pressure and will be watched by the whole $7 trillion offshore banking industry, as it could open further cracks in the secrecy that helped the sector thrive.

UBS agreed on Wednesday to pay a fine of $780 million and to disclose about 250 names of U.S. clients it said had committed tax fraud.

But U.S. tax authorities now wants thousands more names of citizens it says are hiding about $14.8 billion in assets in secret bank accounts.

The civil lawsuit, filed in Miami federal court, also alleges that UBS helped hundreds of taxpayers set up dummy offshore companies to make it easier to avoid reporting obligations under U.S. tax laws.

PRESSURE MOUNTS

Swiss media described the UBS settlement as a capitulation for Switzerland, and experts said they expected it to weaken the country's bank secrecy rules.

A cartoon in Swiss newspaper Tages Anzeiger showed a safe symbolizing bank secrecy being pierced by a bullet carrying the name of the U.S. Department of Justice.

A second cartoon showed a huge rocket, labeled the U.S. tax authorities, coming at full speed while UBS Chairman Peter Kurer was quoted as saying that the bank was no longer a target.

Lawyers say the U.S. investigation will have implications for other offshore centers, which are also coming under pressure from cash-strapped western governments as the economic crisis deepens.

Germany, which suspects thousands of its citizens are hiding money away in offshore accounts, has criticized Switzerland for its bank secrecy laws and paid an informant to obtain names of clients of LGT bank of Liechtenstein, another well-known bank secrecy stronghold.

Prosecutors in Germany are investigating Prince Max of Liechtenstein, who is also LGT's CEO, for possible tax evasion, the Financial Times Deutschland reported on Friday.

LGT said in a statement that Prince Max, who lives in Munich and is the second sun of current ruler Hans Adam II, had complied with Germany tax rules and was cooperating with authorities in the investigation.

(Editing by Will Waterman)

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