A client of Swiss bank UBS has won an appeal to prevent her account data being handed over to U.S. authorities as part of a deal to give details on 4,450 clients to the U.S. tax authority.

The Swiss administrative court said on Friday the existing double taxation agreement with the United States only allowed for data to be disclosed in cases of fraud or the like.

While the pilot case throws doubt on whether Switzerland will be able to hand over all of the 4,450 accounts the United States wants, an alternative criterion allowing for the deal to stand if at least 10,000 clients come forward voluntarily could make this irrelevant.

The client had failed to file a tax form but this did not constitute fraudulent behavior even if large sums of money were involved, the court ruled.

The Swiss tax department would have the opportunity to appeal against the outcome of the test case. A further 25 similar cases were still pending.

The test case is to do with serious and advanced tax evasion. Administrative assistance can only be granted in cases of tax fraud in line with the existing double taxation agreement between the United States and Switzerland, said administrative court spokesman Andrea Arcidiacono.

The question of whether the entire agreement between Switzerland and the United States could be put in danger remained open, Arcidiacono added: It is up to the agreement's parties how to proceed.

A person familiar with the situation told Reuters that a previous December 31 deadline for the 10,000 UBS clients to turn themselves in to tax authorities had been extended by U.S. authorities to a new, undisclosed deadline.

The Swiss justice department said in November that most of the people whose accounts were to be handed over to the U.S. authorities were suspected of serious fraud rather than simple tax evasion.

The department said it would hand over the names of wealthy American clients of UBS with accounts holding more than 1 million Swiss francs ($986,200) where there was a reasonable suspicion of tax fraud.

Accounts of a lesser size, as low as 100,000 Swiss francs, could be included in certain circumstances when there was a scheme of lies identified.

The country would also hand over the names of U.S. citizens holding offshore company accounts with UBS if they were suspected of tax fraud or similar, regardless of whether they were resident in the U.S. or elsewhere.

The submission of information on clients suspected of dodging U.S. taxes by stashing away money in secret accounts, promises to end years of investigation and uncertainty for UBS.

UBS shares were down 5.3 percent by 1512 GMT, against a 3.8 percent dip in the DJ Stoxx European banks index and a 7 percent fall in domestic rival Credit Suisse after U.S. President Barack Obama proposed wide-ranging restrictions on how banks operate.

Swiss financial watchdog FINMA has appealed against a court ruling that it broke Swiss law last year when it ordered UBS to hand over tax-sensitive data of nearly 300 clients to U.S. authorities in an unconnected case.

FINMA's decision was a prelude to the Swiss government bowing to international pressure on tax havens and agreeing to soften its cherished bank secrecy laws in March, a competitive advantage for the Alpine country's banks.

(Additional reporting by Oliver Hirt and Lisa Jucca; Editing by Greg Mahlich)