Trade union Amicus has threatened to end its £8 million fleet contract with Vauxhall if jobs are lost at the company's Ellesmere Port factory.
The union represents workers at the plant where up to 1000 jobs are now under threat as Vauxhall looks to reduce output of the Astra across three plants in Europe.
Bosses at Vauxhall's parent company GM say they are considering cutting a shift at the UK plant to achieve that target.
Amicus says if jobs are lost it will switch its fleet car contract to Nissan or Toyota, which both build more cars than Vauxhall the UK.
General secretary Derek Simpson said: 'Unless GM are prepared to treat decent men and women in Britain with some dignity we will cancel our £8 million contract for Vauxhall cars.'
Vauxhall's Ellesmere Port factory was the fourth most productive in the UK last year with just over 189,000 Astra rolling off the line. Output will fall to around 125,000 if a shift is laid off.
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