Two German unions on Wednesday firmly rejected the idea of a merger between top lenders Deutsche Bank and Commerzbank, even as preliminary talks get underway after pressure from Berlin.

"The merger would not create a 'national champion'" as hoped for in the finance and economy ministries, service workers' union Verdi said.

Instead, the combined banks "would become much more attractive for a 'hostile' takeover, for example from France," the workers' organisation added.

On the jobs front, "at least 10,000 further jobs would be in grave danger" on top of thousands already slated to go as both lenders press through far-reaching restructuring projects, Verdi estimated.

Just as when Commerzbank took over Dresdner Bank in 2009, "massive closures of branches and a staff reduction of at least 15,000" would follow, agreed Stephan Szukalski of the DBV bank workers' union.

While smaller than Verdi, DBV is a force to be reckoned with at Deutsche as it furnishes three supervisory board members and the chair of the bank's works council.

Szukalski said Deutsche's ongoing integration of retail banking subsidiary Postbank made a Commerzbank takeover an "absurd" idea for now.

And given Commerzbank has largely withdrawn from investment banking, Deutsche would find few economies of scale at its flagship division from a merger, he added.

Both Verdi and present and former employees at both banks said cultural differences between the two would hamper integration, with Deutsche workers better paid and more profit-oriented.

Finance Minister Olaf Scholz nevertheless said Monday that "there are talks about the situation as it is" between the lenders, with the federal government a "fair companion" to the discussions.

"I'm especially annoyed that a finance ministry led by the Social Democrats should be exerting such pressure" in favour of a deal, DBV'S Szukalski said.