ROME (Reuters) - German trade union Verdi would oppose a cross-border merger for Commerzbank even if the bidder was not an Italian bank like UniCredit, an official said on Saturday.

Berlin was taken aback by UniCredit's swoop to build a large stake in state-backed Commerzbank, a move the Italian bank says could lead to a merger.

Officials told Reuters on Friday that Germany is working to frustrate a possible takeover that could tie Berlin's fortunes to those of heavily indebted Italy.

"(Our opposition) is not due to the fact that (the bidder) is an Italian bank. It could be French or Spanish," Frederik Werning, a Verdi labour union official and a member of the Commerzbank Supervisory Board, said in a interview with Italian broadcaster La7.

"When a merger happens every time they say that nothing will change but one out of two times they don't keep their promise and jobs would be lost both in Germany and in Italy".

The merging banks would for at least two years be preoccupied with integration at a time when Germany needs to boost investment, Werning added.

"If the takeover happens, UniCredit and Commerzbank will have to take care of themselves for years and they will no longer be strong partners for their clients, neither in Italy nor in Germany," he added.

At the heart of Germany's concern is UniCredit's 40 billion euros ($44 billion) holding of Italian government bonds. Commerzbank, which is smaller and financially weaker than UniCredit, also has billions of euros of Italian bonds.

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