Gabriel, whose Social Democrats (SPD) have blocked a deal between the two governments to levy taxes on German assets in Swiss bank accounts, told Deutschlandfunk on Sunday (12 August) that Swiss banking practices in Germany were comparable to organised crime.
"It's a serious crime," said Gabriel, the chairman of the SPD. "We're talking about organised crime in Swiss banks [operating] in Germany."
Gabriel, who plans to make criticism of banks a centrepiece of the SPD's 2013 campaign against Chancellor Angela Merkel, added the sentence for large-scale tax evasion in Germany could be as long as 10 years in prison.
The Swiss Banking Association rejected his accusation, spokesman Thomas Sutter said.
"It has no basis in fact," he said. "If certain Social-Democrat politicians are prepared to make this kind of accusation, it should be backed up with facts."
Switzerland and Germany struck the tax deal in April, but the ratification is in doubt amid recent purchases by Germany of data leaked from Swiss banks.
SPD promises veto
The SPD has promised to veto the deal in its current form in the upper house of parliament, the Bundesrat. A spokesman for the Swiss government has said the deal would not be renegotiated.
Gabriel said he was upset the German government had not set up a special prosecutor to fight this tax evasion and had not been as tough as the United States in negotiating with Switzerland.
"Why don't we have the courage for that ourselves?" Gabriel said. "Why don't we turn this over to the federal prosecutor to go after it? I'm certain it would put a quick end to all that."
A government spokesperson said Merkel still backed the Swiss-German deal to stem tax evasion.
"The German chancellor and the government think this tax deal is a suitable way to satisfactorily solve this difficult issue between these two countries, both with regard to the past and the future," spokesman Steffen Seibert said today (13 August).
Merkel's government says the deal would enable Berlin to net huge sums if and when it takes effect. Germans hold an estimated €150 billion in Swiss accounts. But the government needs SPD backing for it in the upper house.
Banking secrecy is crucial to Switzerland's €1.6 trillion offshore wealth management industry, and the country has refused to agree to an automatic exchange of information.
The Swiss have reacted angrily to the purchases by German state prosecutors of data from presumed whistleblowers and even issued arrest warrants earlier this year for three German tax investigators they accused of buying secret tax data.