Switzerland has not yet handed over data on clients of UBS to the U.S. authorities as part of a probe into possible tax evasion among wealthy bank customers, a government spokesman was quoted as saying on Sunday.

However, the Swiss government does not know how many UBS customers may have handed over their details to the U.S. tax authorities, Justice Ministry spokesman Guido Balmer told local news agency SDA.

We are working to clarify this, Balmer was quoted as saying, adding the precise number could be made available in the next few days.

A UBS client last week won an appeal to prevent her account data from being given to the United States, a case which raises doubt about the future of the effort to identify U.S. citizens who might have hidden billions of dollars in UBS bank accounts to avoid taxes.

That judgment has thrown the whole deal into doubt and means the United States might not get the details it wants on 4,450 UBS clients.

The United States says it would withdraw its legal summons seeking names of UBS clients if it got account holder names, but it is unclear whether that condition has been met.

A separate U.S. Internal Revenue Service amnesty programme has yielded 14,700 names that the IRS is culling but it is still not clear how many of those are UBS clients.

The Swiss government will discuss next week how to ensure the implementation of the deal after the court's decision. The Justice Ministry was not immediately available to comment on Sunday.

Swiss law gives bank account holders protection against outside scrutiny and UBS's agreement to give names to the United States was seen as a notable departure from that tradition.

(Reporting by Sam Cage; Editing by Greg Mahlich)

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