A further 250 jobs will be cut by Vauxhall at its Ellesmere Port plant on top of the 400 jobs it announced in October to "accelerate the recovery of plant productivity".
The manufacturer, which is now owned by PSA Group, says it will move staff from two production shifts to one.
Unite has called on PSA Group to provide investment guarantees on new models for Vauxhall’s Ellesmere Port plant in Cheshire, which makes the Astra.
Describing the additional job losses as a blow to a world class workforce, which would be felt throughout the supply chain, Unite vowed that it would leave no stone unturned in securing the long term future of the plant.
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. Its current red lines on the customs union and single market will do little to instil business confidence and unlock the investment needed.
“We ask that ministers give PSA and other manufacturers a clear signal that government will do all it needs to support this crucial sector through the Brexit process.
“Unite’s priority is to support our members and protect this plant. We will not tolerate compulsory redundancies and Unite will leave no stone unturned in securing the long term future of Ellesmere Port and the highly skilled workforce. It is one of the jewels in our manufacturing crown and will not be allowed to wither on the vine.”
Vauxhall said it would conduct a statutory 45-day workforce consultation and strive to minimise the impact of the proposed job cuts. It 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.