German labour union IG Metall demanded on Tuesday that industrial company Thyssenkrupp end talks to merge its European steel business with that of India's Tata Steel.
Thyssenkrupp and Tata Steel have been in discussions since July about merging their European steel assets to cut costs and reduce overcapacity, a move that has sparked concerns about job cuts in Germany.
"Management should put its merger plans on ice and discuss with us how the group can make progress," said IG Metall's Detlef Wetzel, who is a member of Thyssenkrupp Steel Europe's supervisory board.
Thyssenkrupp last month unveiled plans to cut costs by 500 million euros ($545 million) at its steel business. IG Metall has said that could lead to 4,000 out of the 27,000 jobs at Thyssenkrupp Steel Europe being axed.
Labour bosses also fear that more cuts could be made at Thyssenkrupp's German steel sites to make room for Tata's ailing steel plant in Port Talbot, Wales, where a deal has been struck to protect jobs and investment.
The German union will stage a rally on Wednesday in the city of Duisburg, home to the headquarters of Thyssenkrupp Steel Europe, and several thousand workers are expected to attend.
"You cannot just say that the UK sites won't be touched but take the axe to Germany," Wetzel said on Tuesday.