Late last month, Zurich-based Julius Baer said client data had been leaked. According to Swiss media, the data was then passed on to officials in North Rhine-Westphalia, a German state which has tapped leaked Swiss data before as part of efforts to hunt tax dodgers.

"We have opened an investigation in this matter," a spokesperson for Swiss prosecutors has been quoted as saying.

According to Swiss media, an IT contractor in Zurich, acting alone, had leaked the data to NRW for an undisclosed payment. He was explicitly tasked with collecting and stealing data from the bank, according to Swiss weekly Handelszeitung. An individual is now in custody, the prosecutor's office said.

A spokeswoman for NRW's finance ministry said the state had "no indication" that its tax inspectors had broken the law.

The acquisition of this data has angered the Swiss authorities, threatening to derail negotiations between the countries over a pact to enable holders of Swiss accounts to clean up their affairs while also preventing future potential abuses.