The German state of Rhineland-Palatinate said on Tuesday it had bought a CD containing data on secret bank accounts which could yield half a billion euros in tax revenues, triggering police raids across the country.
It was unclear who sold the information but in the past, unidentified whistleblowers have provided similar data to German states, including North Rhine-Westphalia.
The purchase comes amid a growing debate in Europe about tax evasion after the leaking of thousands of holders of secret bank accounts and the belated admission by a disgraced former French budget minister that he held a secret foreign account.
The EU's six biggest countries last week agreed to increase their cooperation in fighting tax havens.
"(The data) is authentic and of excellent quality," said Carsten Kuehl, finance minister of Rhineland-Palatinate which is run by opposition Social Democrats (SPD) and Greens.
He added there were 40,000 sets of data which his state had bought for 4 million euros.
"We expect the information to yield tax revenues of around 500 million euros across Germany," said Kuehl in a statement, adding officials had been in touch with investigators and that numerous raids had taken place as a result of the purchase.
The SPD minister said the data proved the high level of criminal energy that goes into tax evasion.
"We must act resolutely against tax fraud. In their investigations, the authorities must use every channel that is legally practicable... Buying tax CDs is part of that. Tax evasion is a serious offence," he said.
Der Spiegel magazine reported that the information was on Germans with accounts in Switzerland and that it affected more than 10,000 customers.
Switzerland said such a purchase would not have been necessary if Germany had implemented a tax treaty, agreed last year but blocked by Germany's SPD in the upper house.
"Data CDs can yield chance finds, at most, and do not clear the way for making sure everyone is taxed, as the treaty that was rejected would have done," said a spokesman for the Swiss finance ministry.
"Switzerland will not provide any official help on the basis of stolen data," he said.
French President Francois Hollande has launched a campaign to crack down on tax evasion and in response to EU pressure, Luxembourg has dropped its opposition to sharing bank data with its partners.
However, Austria is still resisting calls to lift bank secrecy, arguing such a move would amount to an invasion of privacy.
German Finance Minister Wolfgang Schaeuble says Berlin has taken a lead in getting international bodies like the G20 and Organisation for Economic Cooperation and Development (OECD) to define which countries are tax havens and to act against them.
But his attempts to seal a tax deal with Switzerland were thwarted by Germany's SPD who said the agreement was too weak.
The SPD takes a tougher line and thinks cracking down on tax evasion is a winning campaign issue for them, not least because their candidate for chancellor, former finance minister Peer Steinbrueck, has a track record in trying to tackle the issue.
(Additional reporting by Gernot Heller in Berlin and Oliver Hirt in Zurich; Writing by Madeline Chambers, editing by Noah Barkin and Gareth Jones)