The European Commission opened an in-depth investigation to examine the impact of the deal on the companies' customers and consumers. It will decide by Dec. 10 whether to clear the takeover.

"The proposed transaction would bring together the top two suppliers of copper rolled products in an industry that is already highly concentrated," European Competition Commissioner Margrethe Vestager said in a statement.

Aurubis, Europe's biggest copper smelter, wants to sell the unit, which made about 230,000 tonnes of copper and alloyed products last year, and expand into other metals besides copper.

The deal also includes the sale of a 50-percent shareholding in German copper products company Schwermetall Halbzeugwerk, the other 50 percent of which is already held by Wieland-Werke.

Last month, the Commission also opened a full-scale investigation into German copper producer KME's purchase of German peer Mansfelder Kupfer und Messing (MKM).

(Reporting by Foo Yun Chee)