The French company that owns Peugeot and Citroen has struck a deal to buy General Motors' European unit, including Vauxhall.
PSA Group and GM announced the sale ahead of a press conference in Paris.
The deal has raised fears of job losses at Vauxhall's UK factories, which employ 4,500 workers.
With GM's Opel and Vauxhall operations, PSA would become Europe's second largest carmaker, behind Volkswagen but overtaking Renault-Nissan.
Since the two firms announced they were in talks on 14 February, they have met ministers and union officials to address concerns about potential job cuts.
The Unite union said that Vauxhall staff at plants at Luton and Ellesmere Port had endured a "nerve-wracking" few weeks.
Thousands more workers are involved in Vauxhall's showrooms and supply chain.