ZURICH/FRANKFURT German prosecutors are investigating employees of Credit Suisse and its units Clariden Leu and Neue Aargauer Bank on suspicion of helping Germans evade taxes.
The probe was launched in response to findings gleaned from Swiss bank data acquired by officials in the German state Rhineland-Palatinate in early 2013, the prosecutor's office in the city of Koblenz said on Wednesday.
Bank secrecy, which has helped Switzerland become the world's largest offshore centre with $2 trillion in assets, has come under heavy fire since the financial crisis as cash-strapped governments have sought to clamp down on tax evasion.
The Swiss government is considering a possible solution to a long-running dispute with U.S. authorities over Swiss banks accused of helping wealthy Americans evade billions of dollars of tax, a source told Reuters.
The Rhineland-Palatinate purchase comes after a group led by Europe's six biggest countries last week pledged to work together to tackle tax havens, and the belated admission by a disgraced former French budget minister that he held a secret foreign account.
Based on the client data, prosecutors have launched 201 investigations into German suspects, the prosecutor's office said.
The German state on Tuesday said it hoped to yield half a billion euros in lost tax revenues from a CD it purchased containing data on secret bank accounts, triggering police raids across the country.
Zurich-based Credit Suisse spokesman Marc Dosch said on Wednesday: "We have been advising German clients for a long time that they should review their individual (tax) situation and resolve any issues where necessary. If this does not happen, we will terminate our relationship with these clients in the course of the year."
The bank had no knowledge that confidential data had been misappropriated from the bank, he added.
Credit Suisse's increasingly forceful rhetoric on German customers, long courted by Swiss banks, illustrates that it is feeling the pinch following a tax crackdown and several similar data purchases by tax officials.
The move represents one of the first times a Swiss bank has set a specific deadline for clients to either come clean on their offshore funds, or be thrown out.
Credit Suisse, like rivals including UBS and Julius Baer, has been urging clients to deal with undeclared tax funds for roughly two years now, the bank's spokesman said.
The relationship between Switzerland and Germany has been particularly difficult since Germany in December rejected a key tax pact to sweep Swiss accounts clean of tax dodgers.
In 2011, Credit Suisse paid 150 million euros ($196.95 million) to end an investigation of its employees in the German city of Duesseldorf over allegations that they helped citizens dodge taxes. Julius Baer struck a similar deal, while UBS has yet to do the same.
"(The data purchased by Rhineland-Palatinate) is authentic and of excellent quality," state Finance Minister Carsten Kuehl had said on Tuesday, adding there were 40,000 sets of data which his state had bought for 4 million euros.
(Reporting By Katharina Bart and Edward Taylor; Editing by David Cowell)