ZURICH (Reuters) – A Swiss court will decide within weeks whether UBS must hand over historical client data to French tax authorities, a ruling that could open the door to fresh financial claims against Switzerland’s banks.

Although Switzerland exchanges bank account data on current clients with dozens of countries to crack down on cross-border tax cheats, court cases such as UBS’s row with France over client tax avoidance are exposing potentially expensive legacy issues in Europe.

The Swiss supreme court’s ruling could set a precedent whereby other Swiss banks have to hand over confidential historical data to foreign tax authorities that demand it.

The case was triggered by lists of more than 10,000 UBS client account numbers that French tax authorities received from German colleagues after a search of the bank’s local offices.

France wanted names, birth dates and account balances to check whether the account holders had paid their taxes. Swiss tax authorities agreed but UBS fought the case in court.

“It’s quite clear that this ruling has implications beyond (UBS’s) case and we don’t know where it will end,” said Daniel Buehr, a partner at law firm Lalive.

Many other Swiss banks have for years also had active cross-border businesses with clients in France and other European countries which are now hunting down suspected tax evaders who hid money in Switzerland.

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Separately, a French court last month found UBS, Switzerland’s largest bank, guilty of illegally soliciting clients and laundering the proceeds of tax evasion, ordering it to pay 4.5 billion euros ($5.1 billion) in penalties. The bank has denied wrongdoing and is appealing the ruling, but has raised litigation provisions.

Legal experts say the huge fine slapped on UBS could entice other European countries to challenge the Swiss banks, especially if they are forced to hand over historic account data.

Experts point to a similar case in which the supreme court last year granted assistance to German authorities seeking the owner of a Swiss bank account containing 83 million Swiss francs ($82.7 million).

The court ruled that knowing the account number was enough, saying it was not an improper fishing expedition – a key point in France’s case.

If the supreme court rules that UBS has to supply client data to French tax authorities, it could give France new firepower in the dispute over whether the bank aided tax evasion.

UBS is also under investigation in Belgium and Italian authorities have asked Swiss banks for information about their business there. reut.rs/2JbKMI2

“For governments that want to spice up their treasury, this is certainly an interesting option,” said one senior Swiss official.

The case has echoes of when U.S. tax authorities asked UBS for information on the accounts of U.S. clients in 2008.

It eventually complied, triggering extensive investigations of other Swiss banks suspected of helping Americans hide their wealth. UBS eventually settled for $780 million, while payments from all the Swiss banks snared in the probe ran into the billions.

($1 = 0.8833 euros)

($1 = 1.0042 Swiss francs)