IG Metall, Germany's largest trade union called on Thyssenkrupp's management to provide guarantees for jobs, plants and future investments in relation to a planned European steel tie-up with Tata Steel.
In an information leaflet seen by Reuters, IG Metall said management had failed to address employees' concerns about their future once the joint venture goes ahead next year.
"Until now, there have been lots of warm words instead of reliable information. Under these circumstances we cannot approve of this joint venture," IG Metall said, adding it expected management to respond to its demands by Nov. 10.
Thyssenkrupp and Tata Steel in September announced plans for a joint venture that would create Europe's second-largest steelmaker after ArcelorMittal. The merger will also result in up to 4,000 job cuts, although workers fear that will be the tip of the iceberg.
IG Metall also demands that Thyssenkrupp remains a long-term shareholder in the planned 50-50 joint venture and that crucial co-determination agreements will be safeguarded for the merged entity, to be headquartered in the Netherlands.
Labour representatives hold half of the 20 seats on Thyssenkrupp's supervisory board, and while a deal can still be pushed through without their consent, their approval could significantly smooth the transaction.
To appease workers a group of board members and labour representatives was set up shortly after the announcement.
"We take the demands of worker representatives very seriously. They create a basis for negotiations in the joint working group," Thyssenkrupp board member Oliver Burkhard said in an e-mailed statement. "For us, this is an important further step."