Opel's woes continue and the German company has confirmed that they will halt production for a total of 20 days at its main factory in Ruesselsheim and its engine plant in Kaiserslautern between September and the end of the year.

The struggling carmaker said it will cut the hours of workers at the two plants in response to a drop in demand for cars in Europe.

"The European car market is declining substantially. Until recently, the decreased production schedules could be compensated by applying corridor shifts and accrued overtime," said Holger Kimmes, head of personnel at Opel, in a statement. "Now, short work is the right bridging measure."

"Closing down plants for a few days isn't going to help Opel with its most pressing problem of overcapacity," Gommel said.

Ruesselsheim is Opel's biggest production hub and manufactures the Astra and Insignia models.

GM lost $747 million on its European operations last year. A weak economy has hit car sales, forcing carmakers to confront high fixed costs and excess production capacity that GM has said equates to 10 plants.

Opel has 13,800 employees in Ruesselsheim, about half of which will be affected by the measure, the company said. The Kaiserslautern factory has a work force of 2,500 people.