Police snooping in the past makes the issue very sensitive in Germany
Data protection officials in Germany have called for privacy laws to be tightened amid a scandal over illegal sales of personal data.
The officials say they were able to buy six million items of personal data on the internet, including bank account details and phone numbers.
The data cost just 850 euros (£671), the officials said.
Call centres and lottery firms are suspected of trading in the data. Prosecutors are now investigating.
A data protection office in Schleswig-Holstein, north Germany, said it had received CDs this month containing thousands of personal data items collected by a call centre, including bank account details, dates of birth and addresses.
The head of the office, Thilo Weichert, told Germany's Sueddeutsche Zeitung newspaper that up to 20 million pieces of data from people's bank accounts had been sold on to third parties.
"The sale of bank account data is just as illegal as unsolicited telephone marketing - so-called 'cold calling'," Mr Weichert said.
"Lawmakers can do more to protect consumers. The transfer of data for marketing purposes should be made universally conditional on the customer's approval," he said.
Firms that discover illegal use of customers' data should be obliged to inform the affected customers, he added.
Sebastian Edathy, a Social Democrat (SPD) parliamentarian, urged parliament to "find a quick response to these blatant cases of abuse".
Germans are especially sensitive about data privacy in light of the widespread snooping by secret police in the Third Reich and former communist East Germany.