Press reports said that an informer had offered Berlin the names of up to 1,500 Germans hiding their riches from the tax authorities in Switzerland for €2.5 million ($3.5 million).

Switzerland warned Germany that buying stolen information “violates public policy and the principle of good faith … (and) constitutes a breach of the privacy of the clients concerned,” refusing to cooperate if Berlin went ahead.

But with reports saying that the information would give Germany around €100 million in recovered taxes, German Chancellor Angela Merkel said “everything must be done to get hold of these data.”

“Like every sensible person, I want to clamp down on tax evasion,” Merkel told reporters in Berlin. “If these data are relevant we should aim to get hold of them.”

The Finance Ministry said that Germany was considering following a 2008 “precedent” involving Liechtenstein, another Alpine country that has come under fire for its banking secrecy.

In that case, the German secret service handed over €5 million for the names of hundreds of German business executives, sports stars and entertainers allegedly hiding some €4 billion. In the ensuing investigation, Germany clawed back some €180 million.

The scandal claimed the scalp of Deutsche Post chief Klaus Zumwinkel, who got a two-year suspended jail sentence and was fined close to €1 million after admitting hiding cash in the principality.

Germany handed other countries the names of some their nationals and the scandal put tax havens in the international firing line just as the financial crisis sparked new calls for transparency in the banking industry.

Switzerland and Liechtenstein have since moved to clean up their acts, agreeing to share more tax information with other countries. Both have since been removed from an OECD “grey list” of tax havens.

Swiss banking giant UBS last year agreed to hand over details of about 4,450 clients and US taxpayers, although last month a Swiss court upheld an appeal by one taxpayer against the transfer.

There are some voices in Germany, however, that are cautious about paying for the information and with a new government in power after elections late last year, it is not clear that Berlin would cough up again.

“Personally, I have a problem with it if one hands over money for something that has come into someone’s possession in a legally questionable fashion,” Defence Minister Karl-Theodor zu Guttenberg, a close Merkel ally, told the Swiss daily Neue Zürcher Zeitung.

Doubts emerged meanwhile about the identity of the whistle-blower.

The Financial Times Deutschland daily named him as Herve Falciani, a IT specialist formerly employed by British bank HSBC in Geneva, who passed on data to the French tax authorities.

“This is just a rumour,” Falciani, 37, told news agency AFP. “If they have any shred of proof, they should produce it.