The prosecutor’s office in Düsseldorf said that about 150 investigators took part in searches as part of a probe into allegations that bank staff assisted clients to dodge their taxes.

State attorney Johannes Mocken said the raids “targeted Credit Suisse staff suspected of having assisted tax fraud by clients.”

The Swiss bank said in a statement that it was “cooperating with the relevant authorities” in Germany but added: “As it is an ongoing investigation, we cannot provide more information.”

German officials in the state of North Rhine-Westphalia purchased a CD in February that reportedly contained information on more than 1,000 wealthy Germans who could be concerned by the investigation.

Those with accounts at Credit Suisse have been urged to come forward of their own accord to avoid prosecution.

A spokesman for prosecutors told news agency AFP in March that “the Credit Suisse clients have investments in total of around €1.2 billion.” The amount of tax owed to the authorities was unclear, he added, but according to several sources they stood to recover up to €400 million.

Swiss authorities have complained about the acquisition, and the affair has provoked diplomatic tension with Germany, which is determined to recover tax revenue from accounts in neighbouring countries that might shield funds.

German press reports have said the state paid €2.5 million ($3.2 million) for the information.

In 2008, a similar deal netted a long list of names and bank accounts in the principality of Liechtenstein which let officials recover around €200 million in unpaid taxes and led to the arrest of the head of the logistics group Deutsche Post.

The saga put Liechtenstein and other tax havens including Switzerland in the firing line of international efforts against offshore banking havens and tax dodgers.