Magna, the Canadian parts manufacturer which will become the new owner of Vauxhall and Opel, has confirmed it could cut 10,500 jobs across Europe.
Magna’s co-chief executive Siegfried Wolf said the cuts could take up to a year to implement.
Opel employs a total of 54,500 workers across Europe, with 25,000 based in Germany.
British unions have expressed concern about the long-term future of Vauxhall's 5,500 UK workers and its two British plants in Luton and Ellesmere Port.
Magna has also suggested shifting some production from a plant in Zaragoza in Spain back to Germany.
Tony Woodley, Unite's joint general secretary, said: "The uncertainty surrounding the ownership of Vauxhall is now over, but the uncertainty surrounding the long term future of Britain's plants will continue.
"One of the alternatives could have been the unthinkable position of liquidation. Nevertheless, with Magna as the new owner , we need to make sure that British plants and people are not treated disproportionately during the restructuring that will take place. With that in mind, the union and the Government will no doubt continue to negotiate with Magna.
"We expect financial support from the UK government for Magna to be dependent on the job and plant commitments given by the company."