Economics Minister Brigitte Zypries said it was "unacceptable" that GM and PSA had not informed the Opel works council, the union IG Metall and the regional and federal government of their plans.
Senior Opel board members were not aware of the advanced state of GM negotiations with PSA regarding the potential sale of Opel to the French company, according to German magazine Der Speigel.
GM is reportedly seeking a multi-billion dollar amount for Opel. Still, GM Europe failed to break even in 2016.
Union Unite is calling for urgent talks with General Motors after it was revealed the PSA Group, owners of French vehicle brands Peugeot and Citroen, is in talks to take over the GM European brands.
The maker of Peugeot, Citroen and DS cars is "exploring a number of strategic initiatives with GM with the aim of increasing its profitability and operating efficiency, including a potential acquisition of Opel", a spokesman said.
"The Government remains in close contact with GM as we closely monitor the situation".
PSA also said its chief executive Carlos Tavares planned to meet senior German officials, possibly including Mrs Merkel, in the near future.
A combination between the two would have around 16% of the European vehicle market, and would take PSA (pugoy) above its French rival Renault (rnsdf). In 2016, the French auto maker;s three brands, Peugeot, Citroen and DS, booked 1,446,052 sales, while Opel reported sales of 979,427.
Vauxhall UK also has headquarters in Luton. One scenario already raised by General Motors sources in discussions with Autocar is the possibility of a sweeping consolidation of Vauxhall operations in the United Kingdom, with the brand name resigned to history and being replaced by Opel - a move that was seriously considered by General Motors following the financial crisis of 2009 but ultimately decided against. The French government holds a stake in PSA.
Selling Opel and Vauxhall, which added nearly one million cars to its sales, could mean abandoning the global volume race in which it is ranked third behind Volkswagen and Toyota, with just over 10 million vehicles delivered previous year.