Swiss data protection law was violated in the Swift affair - in which international banking transaction details were revealed to the United States - says a top official.
Hanspeter Thür, the Federal Data Protection Commissioner, stated on Friday that Swiss banks should have informed their customers of the move.
It was revealed in July that the US Treasury had been monitoring records - including Swiss ones - at the Belgium-based Society for Worldwide Interbank Financial Telecommunication (Swift) for the past five years. This surveillance started following the September 11 attacks on the US.
Swift is a cooperative owned by over 7,800 financial institutions that operate from more than 200 countries. In Switzerland 99 banks and 254 other institutions are connected to Swift, with daily transactions worth around SFr200 billion ($160 billion).
In a statement, Thür said that banks and data protection bodies should have informed customers making international money transfers via Swift that their data could be passed on to third parties.
He added that just the possibility of the data being leaked should have been grounds enough to flag the issue to customers. Thür also warned that there had been no confirmation that the American authorities were no longer receiving transaction details.
When the news broke, Switzerland's biggest banks, UBS and Credit Suisse, played down the news by saying the CIA probe only related to limited data.
UBS and the Swiss Bankers Association both told swissinfo at the time that the surveillance did not compromise Swiss banking secrecy. This was also the view of the government.
In his statement, Thür said that issue was whether data could be handed over to countries that did not have the same data protection rules as Switzerland.
He called for measures to be taken and to find a solution that takes into account the US law as well as European norms.
The commissioner said he had been prompted to take a position on the issue following a report by a Belgian data protection body, government's statement on the matter and its own investigation. He has previously stated his alarm over the Swift issue.
The Swiss finance minister, Hans-Rudolph Merz, has always maintained that the country's cherished banking secrecy was not harmed in the affair.
The government said this was because the probe did not take place in Switzerland but in the US and in Belgium and that banking secrecy was only applicable in Switzerland. There was therefore no need to take action, it said.
swissinfo with agencies
Swift (Society for Worldwide Interbank Financial Telecommunication) is a cooperative founded in 1973 and headquartered in Brussels, Belgium.
The consortium operates a secure electronic messaging service used by over 7,800 financial institutions – including banks, brokerages and investment managers – to communicate with their counterparts in more than 200 countries.
A spokesman said the network handles some nine million transfer instructions and confirmations a day with a value of about $6 trillion (SFr7.5 trillion).
Two Swiss sit on the 25-man board.