PSA Group, the French company that owns Peugeot and Citroen, has reached a deal to buy General Motors' European unit, which includes Vauxhall and Opel, it was revealed on 6 March.
PSA will become Europe's second biggest carmaker behind Volkswagen upon completion of the €2.2bn (£1.9bn) deal. The company has pledged to achieve €1.7bn in cost savings from the acquisition by 2026.
PSA will pay about €1.13bn in cash for Opel, with an additional €650m to be financed by issuing to GM warrants in PSA shares, which can be exercised after five years.
The group, alongside French bank BNP Paribas, will pay an extra €900m for Opel's financing arm, which will operate as a joint venture.
"We are confident that the Opel/Vauxhall turnaround will significantly accelerate with our support, while respecting the commitments made by GM to the Opel/Vauxhall employees," said Carlos Tavares, chairman of PSA's managing board.
GM chairman and chief executive Mary Barra added the deal would put Opel and Vauxhall "in an even stronger position for the long term."
"We are reshaping our company and delivering consistent, record results for our owners through disciplined capital allocation to our higher-return investments in our core automotive business and in new technologies that are enabling us to lead the future of personal mobility," she said.
GM's Opel and Vauxhall brands have not turned a profit since 1999. The Detroit-based group had targeted returning to profitability in Europe in 2016, but ended up posting a loss before interest and tax of $300m (£240m) last year.
However, the proposed deal could threaten thousands of jobs at Vauxhall's factories in Ellesmere Port in Cheshire and Luton in Bedfordshire. Vauxhall's UK workforce measures 35,000 and includes 23,000 in retail and 4,500 at factories at Ellesmere Port and Luton.
Len McCluskey, general secretary of Unite, said the union would work to "ensure a long-term future for our plants and the tens of thousands of workers depending on them", although it conceded initial negotiations with PSA had been "relatively positive".
"We will also be urging the government to stay at the table, just as the French and German governments do, to provide full support for our auto workers through this deeply unsettling time," he added.
Ian Lucas, MP for Wrexham, whose constituency is close to the Ellesmere Port plant, said news of the announcement made this a "very difficult day, especially if you work at Vauxhall".
"In the next few years we've really got to make the case for Ellesmere Port and for more investment in the UK car industry," he added.
On Sunday Prime Minister Theresa May and the Business Secretary Greg Clark spoke to Barra, having previously held a conversation with Tavares in which they discussed their shared desire to protect and promote the jobs at the Ellesmere Port and Luton plants.
"The Prime Minister set out to Ms Barra the importance of the Vauxhall brand to the UK and reiterated her desire for the jobs at both plants to be secured for the long term," aDowning Street spokesperson said.
"Ms Barra made clear that Vauxhall would remain a British brand and that the deal would recognise and respect all agreements regarding the workforce.
"Both the Prime Minister and Ms Barra expressed their confidence that the deal had the potential to strengthen the Vauxhall brand and allow for further growth, supported by the Government's Industrial Strategy and the continued strength of the automotive sector in the UK."