Doubts have emerged about the identity of the informant whose tip to Germany's BND intelligence service in 2006 sparked the country's biggest-ever tax evasion crackdown.

Underlining the controversial nature of the agency's involvement, Pierre Mirabaud, president of the Swiss banking federation, apologised yesterday for describing Germany's methods as "reminiscent of the Gestapo".

Liechtenstein's prosecutors and LGT Group, whose German clients are at the centre of the tax inquiry, suspect Heinrich Kieber, a former employee convicted of fraud in 2004 who has now disappeared, to be the source of the leak. Yet many point to facts suggesting Mr Kieber may not be the man who offered the BND a list of 750 to 1,000 suspected tax evaders in January 2006.

"The information includes documents that we are pretty sure Kieber never had access to," said one German government official.

Mr Kieber, who worked for LGT Treuhand, a subsidiary of the group, between 2001 and 2002 as an interpreter, could not be located for comment.

Backed by the chancellery, the finance ministry and the tax authorities, the BND acquired a DVD containing the data for €4.2m in 2006 and passed it to prosecutors in North Rhine-Westphalia, leading to last week's raids.

The identity of the source matters because of the implications for LGT and Liechtenstein. If Mr Kieber were not the informant, it would mean security measures put in place by LGT after his arrest failed to prevent another leak.