Workers at German factories of carmaker Volkswagen, which has been plagued with crisis lately, are set to go on strike from Monday over plans to cut thousands of jobs, union IG Metall said.

"If need be, it will be the toughest collective bargaining battle Volkswagen has ever seen," the union said on Sunday.

The crisis-wracked auto-titan has been locked in bitter talks with unions since announcing in September that it was mulling the unprecedented step of shuttering plants in Germany, where it has some 120,000 employees.

VW has been hit hard by high manufacturing costs at home, a stuttering shift to electric vehicles, and tough competition in China, a key market.

Ahead of talks last month, the union and VW's works councils put forward a series of proposals they said would save 1.5 billion euros ($1.6 billion) in labor costs without the need for site closures.

These included proposals for management and staff to waive bonuses. The union also said it could drop a demand for pay raises in exchange for working shorter hours at some factories.

But Volkswagen said it had concluded that, while the measures could help in the short term, they would not lead "to any long-term financial relief for the company in the coming years."

IG Metall at the time described VW's response as "extremely regrettable," accusing it of "ignoring the constructive proposals of the employee representatives."