Derek Simpson, Amicus general secretary, said: "Unless General Motors 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 and explore options for sourcing our car fleet from them."
Amicus wants reassurances from Vauxhall parent company GM about the future of 1,000 jobs at Ellesmere Port, which it is considering cutting to meet its restructuring targets throughout Europe.
GM said it wants to improve efficiency at the site, but no decision has been reached about its future.