Vauxhall is cutting another 250 jobs at its Ellesmere Port car plant on top of the 400 jobs it announced in October.
The carmaker, now owned by France's PSA Group - maker of Peugeot and Citroen - said it needed to "accelerate the recovery of plant productivity".
The Ellesmere Port plant in Cheshire, which makes the Astra, will move staff from two production shifts to one.
The company told the Unite union last week that more voluntary redundancies were now needed.
Unite general secretary, Len McCluskey, said: "This is an additional blow to a world class workforce that is one of the most efficient in the industry. PSA must provide investment guarantees on new models for Ellesmere Port as a matter of urgency.
"The government must play its part too and provide guarantees on frictionless trade after Brexit."
Vauxhall said it would conduct a statutory 45-day workforce consultation and strive to minimise the impact of the proposed job cuts.
The company also said it was committed to keep making the Astra at Ellesmere Port.
PSA has said previously that manufacturing costs at Ellesmere were higher than other plants it owned.
Vauxhall employs about 4,500 people in the UK, with about 1,800 at Ellesmere Port. The company also has a factory at Luton, which makes vans.
In August last year, PSA became Europe's second-biggest carmaker after Volkswagen when it completed the purchase of Vauxhall and German brand Opel from General Motors.