Amicus is threatening to switch its £8m car contract from Vauxhall to one of the Japanese carmakers with a British plant if General Motors' British subsidiary makes substantial cuts in the workforce at its Ellesmere Port plant in Merseyside.

The union will also encourage its one million members and their families, as well as other trade unions, to boycott Vauxhall as part of its retaliation against any heavy job losses. Last week workers in Merseyside staged a wildcat strike after comments by the head of GM Europe, Carl-Peter Forster, were seen as heralding redundancies at the plant.