Rival AUB, a small group that positions itself as an alternative to a traditional union, is considered more "employer friendly," Germany's Deutsche Welle broadcaster reported.
"We have the suspicion and indications for the fact that the AUB was financed by Siemens in order to build up a kind of anti-trade union in opposition to IG Metall," IG Metall union leader Juergen Peters said.
Last week Siemens central management board member Johannes Feldmayer was arrested on suspicion of having paid bribes to an AUB official. Former Siemens finance chief Karl-Hermann Baumann is also being investigated in the matter, Deutsche Welle reported.
The men are alleged to have paid $20 million to $27 million to AUB chief Wilhelm Schelsky, who was arrested in February.
Chief Executive Klaus Kleinfeld said he considered IG Metall Siemens' "partner when it comes to wage talks," adding he did not "have the impression that the AUB is particularly employer-friendly," Frankfurt's Allgemeine Zeitung reported.
German prosecutors call the Siemens-AUB investigation "Operation Amigo," Deutsche Welle said.