Swiss prosecutors said Thursday they had opened an investigation into a massive leak last year of Credit Suisse account records, which a media probe alleged proved the bank held billions in dirty money.

The Office of the Attorney General of Switzerland (OAG) confirmed media reports that it had received a "criminal complaint" in the so-called "Suisse Secrets" case.

"In this connection, the OAG is conducting criminal proceedings on suspicion of economic intelligence, violation of business secrecy and violation of banking secrecy," it said in an email to AFP.

The OAG said that since economic intelligence was considered a political crime, it had sought and been granted authorisation from the justice ministry to pursue a criminal case.

It did not say who had filed the criminal complaint, but according to the Swiss investigative news site Gotham City, it came from Credit Suisse itself.

When contacted by AFP the bank said it would "not comment on ongoing investigations".

The leaked records on more than 18,000 Credit Suisse accounts dating back to the 1940s were used in an international media investigation published in February 2022.

That investigation, which was coordinated by the Organized Crime and Corruption Reporting Project (OCCRP), said the data revealed how Switzerland's second largest bank held more than $8 billion in accounts of criminals, dictators and rights abusers.

Credit Suisse has flatly rejected the "allegations and insinuations" in that probe.

While the media investigation focused on the bank's alleged handling of dirty money, it also shone a spotlight on Article 47 of Switzerland's Banking Act, which critics say has weakened media freedom in the country.

Experts say the 2015 law, which made it a criminal offence to reveal leaked banking data punishable with up to five years in prison, effectively silences insiders or journalists who may want to expose wrongdoing within a Swiss bank.

While 48 media companies from around the world participated in the Suisse Secrets investigation, no Swiss news media took part due to the risk of criminal prosecution.

The United Nations' top expert on freedom of expression, Irene Kahn, wrote to the Swiss government last March to voice her concerns about the law, and warned in an interview with the Tages-Anzeiger daily that it was "an example of the criminalisation of journalism."