"In consultation with the works council and the IG Metall labour union, Adam Opel AG will introduce short-time work at its plants in Rüsselsheim and Kaiserslautern from September," Opel said in a statement, adding that 20 working days would be cut between then and the end of the year.
"The European car market is dropping dramatically," the carmaker complained.
"Falling capacity utilisation can no longer be compensated with measures such as flexitime. Short-time work is now the right way to bridge this weakness in the market," said Opel's personnel chief Holger Kimmes.
Under short-time work schemes, employees see their working hours reduced for a limited period, but the state, in the form of the Federal Labour Agency, partially makes up for the corresponding shortfall in pay.
The head of the general works council, Wolfgang Schäfer-Klug, said the measure would "secure jobs" and help limit financial hardship for the employees concerned.
The measure would be introduced both in the production operations, as well as in administration.
Opel has a total four production sites in Germany: alongside Rüsselsheim - which is the main site as well as the group's headquarters - and Kaiserslautern, it also has plants in Bochum and Eisenach.
Rüsselsheim employs a workforce of 13,800, with 3,500 in production, 3,300 in administration and a further 7,000 in engineering, with "around half" of the employees there to be affected, Opel said.
The Kaiserslautern plant employs a workforce of 2,500.
GM sustained a loss of $400 million from its European operations in the second quarter of this year, as the unit battles with the eurozone sovereign debt crisis and massive overcapacity issues.
At the end of June, Opel's supervisory board approved deep restructuring, massive investment in the product range of the Opel and Vauxhall brands, and a new marketing strategy.