In its decision, the Alpine country's Federal Supreme Court rejected an appeal by Credit Suisse clients and gave its blessing to a request made in 2011 by Washington for data on clients suspected of evading US taxes.

At the time, Swiss tax authorities gave the go-ahead to temporarily circumvent Switzerland's long-sacrosanct bank secrecy laws, but the clients in question have been challenging the decision through the Swiss court system.

The federal court on Friday ruled that Washington's broad request "was not a fishing expedition" even though the request did not specifically name the suspected clients.

It said the US demand was "detailed enough to show grounds for suspecting tax fraud or similar offences and to allow the identification of the wanted persons."

The US request for assistance concerned a company with an account declared to Swiss tax authorities, but whose beneficiaries all lived in the United States.

The court ruled that the clients' decision to open an account through a firm not subjected to US taxation could be considered an attempt to dodge US taxes.

A spokesman for the Swiss finance ministry, which has been sitting on the bank data while waiting for the final court ruling, told AFP Friday that the information "can now be transmitted" to the Americans.

Swiss banks are believed in the past to have accepted billions of dollars belonging to American citizens who have not declared these assets to US tax authorities, though they now refuse such money.