With many carmakers and suppliers choosing to build new factories in eastern Europe, "this is sending out the wrong signal at the wrong time," Forster told Automobilwoche.
According to Opel personnel manager Norbert Küpper's calculations, a sudden cut in Eisenach working hours from the current 38 hours a week to 35 hours, together with an additional wage increase asked for by the unions, would increase costs at the plant by 8.4 percent.
"It would be difficult to compensate for that cost increase by raising productivity," said Forster. "However, I am not going to threaten to move our production to the east."
But Forster said he might be forced to review future investments at Eisenach. "Wage levels would also play a central role," he said.
Strike ballots are currently being held at Eisenach.