Norbert Walter-Borjans, finance minister of western North Rhine-Westphalia (NRW) state, told ZDF public television that the purchase of the CDs was “an important part” of its tax fraud investigations.

Media reports earlier in the week said the region, Germany’s most populous, had bought more data after previous similar purchases incurred the anger of the German federal and Swiss authorities.

Berlin and Bern want to ban the practice under a new treaty that is still in the process of being ratified.

Walter-Borjans declined to comment on the reports or specific cases but said NRW state had so far spent a “one-digit amount in the millions” on buying the data.

This in turn had reaped “approximately 300 million” euros of evaded taxes, he added.

Germany and Switzerland became embroiled in a major row in 2010 when German authorities raided branches of Credit Suisse bank in 13 German cities after buying data on suspected tax dodgers.

Switzerland reacted angrily, saying the data — bought for a reported 2.5 million euros — was stolen in violation of its banking secrecy laws.