One of Vauxhall's biggest unions has threatened to boycott its cars and buy Japanese instead if the company makes big job cuts at its Ellesmere Port plant on Merseyside or closes the factory altogether.
Amicus said it would cancel its £8m contract with the General Motors subsidiary and urge its one million members to stop buying Vauxhall cars as well. Members of other trade unions would be encouraged to do likewise.
The move came as negotiations continued over the future of Ellesmere Port amid fears that GM intends to axe about a thousand jobs at the plant and reduce production there of the Astra.
The prospect of Ellesmere Port being reduced from three shifts to two has raised fears among unions that its long-term future is in doubt when the Astra is eventually phased out.
The president of GM Europe, Carl-Peter Forster, provoked union fury last week by admitting it was considering cutting the workforce at Ellesmere Port because it was easier to sack workers in Britain than on the Continent.
Derek Simpson, the union's general secretary, said: "Unless GM are prepared to treat decent men and women in Britain with some dignity, we will cancel our £8m contract for Vauxhall cars. We will encourage our members to buy their cars from a manufacturer who supports the British economy and urge other unions to do the same. We will be meeting with Japanese manufacturers who employ UK workers to explore options for sourcing our car fleet from them."
Should Ellesmere Port close, the only remaining volume car makers in Britain would all be Japanese. Nissan, Toyota and Honda produced more than 600,000 cars between them in the UK last year.
Workers at Ellesmere Port walked out last week in a wildcat strike after Mr Forster's comments, made at the opening ceremony of a new design centre in Germany. The factory's 3,250-strong workforce produced 188,800 Astras last year - about a third of all the Astras made in Europe. Unions fear that GM is about to cut out one of three shifts at the plant, with the loss of 1,000 jobs.
This would represent a fresh blow to the UK motor industry after Peugeot's recent decision to close its Coventry plant with 2,300 job losses, and the closure last year of MG Rover's Longbridge factory and Jaguar's Coventry site.