"You are putting my life in danger," Heinrich Kieber wrote to German intelligence services, according to German weekly Focus in an article released in advance of publication over the weekend.
The informant has blamed the intelligence services for not keeping his identity secret and asked them to provide him with a new identity so that he can relocate to South America. His request has been refused, Focus reported.
Der Spiegel magazine reported that Kieber fears repercussions after handing over names of members of the Saudi royal family and relatives of former Indonesian dictator Suharto, who died in January.
The German government last month admitted paying more than €4 million ($6 million) to an informer for bank data that led to the biggest tax fraud probe ever in Germany and sparked similar investigations around the globe.
Liechtenstein's LGT bank claims that tax inspectors are working from a list with the names of 1,400 of its clients - 600 of them German - that was stolen in 2002.
According to Der Spiegel, 134 clients have turned themselves in to authorities in hopes of reducing potential sentences. They include prominent figures in the textile and cosmetic industries, the magazine said.
Liechtenstein's Crown Prince Alois has also said he is prepared to impose taxes on German foundations in the principality and hand over the revenue to Germany, Focus said. Such a measure could provide an average of between €800 million and €1 billion to Germany each year.
German concerns have been focused on minimally taxed trusts based in the principality believed to facilitate tax evasion among Europe's wealthy set.
Austria, Australia, Britain, Canada, France, Greece, Italy, New Zealand, Spain, Sweden and the United States have also launched investigations into their citizens' investments in Liechtenstein.
The tiny tax haven - wedged between Austria and Switzerland - is now facing intense pressure to reform its lucrative financial sector and help fight fiscal fraud.