By Erik Kirschbaum / BERLIN |Aug 12 (Reuters) - The leader of Germany's centre-left opposition, Sigmar Gabriel, criticised Swiss banks for helping Germans avoid taxes, giving a strongly worded radio interview on Sunday that could put new strains on ties between the two neighbours.
Gabriel, whose Social Democrats (SPD) have blocked a deal between the German and Swiss governments to levy taxes on German assets in Swiss bank accounts, told Deutschlandfunk that Swiss banking practices in Germany were comparable with organised crime.
"It's a serious crime," said Gabriel, the chairman of the SPD who may run against Chancellor Angela Merkel in the 2013 election. "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, added the sentence for large-scale tax evasion in Germany could be as long as 10 years in prison.
The Swiss Banking Association completely rejected his accusation, spokesman Thomas Sutter said.
"It has no basis in fact," he added. "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.
The SPD has promised to veto the deal in its current form in the upper house of parliament, while a spokesman for the Swiss government said on Thursday it 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."
On Saturday, another SPD leader, Andrea Nahles, said the deal to levy taxes on German assets wasn't worth the paper it was written on and should be scrapped.
Chancellor Angela 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 euros ($184.7 billion) in Swiss accounts. But the government needs SPD backing for it in the upper house.
Banking secrecy is crucial to Switzerland's $2 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.
($1 = 0.8121 euros) (Reporting By Erik Kirschbaum, with reporting by Katharina Bart)