A walkout by staff at Vauxhall's Ellesmere Port factory, over news that parent firm GM is considering cutting 1,000 jobs, continued into the night.
The union wants job cuts to be spread out
The dispute has stopped production at the plant in Cheshire.
GM confirmed it was in talks about improving the efficiency of the site, but said no decision had yet been made.
Negotiations between management and unions at the plant broke up without agreement on Thursday night and are due to start again on Monday morning.
"Whatever the decision will look like, GM Europe will handle this as socially responsibly as possible," the carmaker said in a statement.
"For the future, it is important for Ellesmere Port that everyone works together to meet the continuing demand for the Astra.
"We look forward to working together to give the plant the best opportunity to compete in the automotive industry in the long term."
'Easiest to sack'
The union claims GM Europe President Carl-Peter Forster said the company wants to cut one shift from the plant, which it says would lead to 1,000 job losses, representing about a third of the plant's workforce.
"We would have expected better from such a senior executive of the company," said Roger Maddison of the Amicus union.
"His comments are extremely unhelpful during critical ongoing negotiations between the company and trade unions in Germany."
Tony Woodley, general secretary of the Transport and General Workers Union said "British car workers are among the best in Europe, but they're the easiest to sack".
"These decisions are not led by business logic, but by the fact that our laws don't protect our workers," he added.
A day before the walkout, Amicus has expressed its opposition to the UK being singled out for job cuts saying it made "no sense".
Instead Amicus said it wanted cuts to be spread throughout Europe's Astra plants in Belgium and Germany.
Amicus said it had put several proposals to the firm which were still being considered, in an attempt to avoid job cuts.
General Motors is assessing production levels at three Astra factories in Europe.
Aims to reduce costs are part of the ailing car firm's wider restructuring plans, which include cutting 12,000 jobs across the continent.