A top Swiss court ruled Switzerland may not hand over the bank details of a Credit Suisse client to US tax authorities, in a potential setback to solving a dispute between the two countries.

Switzerland and the US have for years been locked in a conflict over the fact that wealthy Americans are dodging taxes by hiding money in Swiss accounts. Washington is pressuring banks in Switzerland to divulge their names and financial details.

In a ruling published on Wednesday that cannot be appealed, Switzerland's Federal Administrative Court said the tax office was not allowed to hand over information on a Credit Suisse client to the US Internal Revenue Service because its request was based solely on a suspicion of tax evasion and did not include the bank account holder's name.

Unlike most countries, Switzerland has distinguished between tax fraud, which is illegal, and tax evasion, which is not. Under a new deal, which has yet to be ratified by the US, Switzerland would assist US authorities in cases of tax evasion as well as tax fraud.

Eleven Swiss banks including Credit Suisse and Julius Baer are under investigation in the United States for aiding US citizens who are suspected of dodging taxes.

Switzerland is trying to get the investigations dropped in return for the payment of fines and the transfer of US client names and is also seeking a deal to shield the remainder of its 300-odd banks from US prosecution.

Flagship bank UBS in 2009 was forced to pay a fine and release the names of 4,500 clients to US officials as the government was forced to bend Switzerland's long-cherished secrecy laws, which have underpinned its large financial sector.

The deal over UBS data was later upended by a Swiss court, citing a breach of bank secrecy, forcing the government to turn to parliament to push the deal through.

In an attempt to avoid a repeat of such legal wrangles holding up a new deal, the Swiss parliament approved in February a plan that would allow US officials to request information on suspected tax cheats based solely on their behaviour rather than name or bank account number.

A spokesman for Credit Suisse declined to comment and said the Zurich-listed bank had not been a direct party in the case.-Reuters