Unite's general secretary, Len McCluskey, has warned that the union "will not accept any job losses or plant closures" if Vauxhall is sold to the company behind Peugeot and Citroën.
That failed experiment led PSA to seek a bailout from the French government and Chinese carmaker Dongfeng Motor, which also holds a 14% stake in the firm.
However, despite Neumann's plan, Der Speigel claims that even he was surprised at how advanced the talks were because GM had promised they would put his electric vehicle brand ideas to vote at board level by May.
PSA Group confirmed Tuesday that it is "exploring numerous strategic initiatives aiming at improving its profitability and operational efficiency, including a potential acquisition of Opel", GM's Germany-based division.
Detroit-based GM has been trying to return its European business to profit.
German ministers will discuss the deal with the French government in an effort to limit the potential impact on German workers, Chancellor Angela Merkel's spokesman said.
GM Europe has been a drag on the automaker's global profitability since 1999, the past year Opel and Vauxhall recorded a net profit.
Bertrand Blaise, executive VP for corporate communications at Group PSA, is quoted by the news agency as saying, 'We are in discussions with Opel to expand upon our existing projects, ' but declined to comment further. Selling to PSA is a lower risk in that sense because the two companies already share some engine and chassis technology.
It would overtake Renault to become the second largest European carmaker, after Volkswagen.
In 2016 Opel Vauxhall had a western Europe market share of 6.6%, while Peugeot, Citroen and its DS upmarket subsidiary had just over 10 per cent, according to Automotive Industry Data. "This is my clear expectation regarding General Motors".
Peugeot - which also controls the Citroen marque - is part-owned by the French government, which holds a 14pc stake after a state bail-out deal. It had hoped to reach break-even by now, but last year posted a loss of $257 million for the year.