FRANKFURT (Reuters) - Germany's most powerful union has threatened to withhold support for a partial sale of Thyssenkrupp's steel unit unless the group provides guarantees for investments, sites and jobs, a leading labour representative told weekly Welt am Sonntag.

"Those are red lines for us," said Knut Giesler, who leads IG Metall's branch in North Rhine-Westphalia, where Thyssenkrupp is based.

If there were no assurances in writing by a supervisory board meeting scheduled for May 23, Giesler could not imagine that labour leaders, which hold half of the committee's seats, would vote in favour of a planned sale of 20% of Thyssenkrupp's steel unit to Czech billionaire Daniel Kretinsky, he said.

In case of a stalemate labour representatives can still be outvoted by Supervisory Board Chairman Siegfried Russwurm, whose vote counts double in such a case, but Giesler warned that this could worsen an already tense relationship with management.

"Mr Russwurm and the supervisory board should think carefully about whether they really want to override the strongest co-determined area in the company with a double vote," Giesler said.

"If they want to deal with us in this way, we will make sure there is a fitting response from the workforce," he said. "The Group has to decide whether it wants months of unrest in the workforce and at the sites."

Labour representatives have harshly criticised the style of Thyssenkrupp CEO Miguel Lopez, accusing him of not involving workers in negotiations and failing to honour long-standing traditions that give much influence to unions at the group.

(Reporting by Christoph Steitz, Editing by Louise Heavens)