A union representing workers at Vauxhall's Ellesmere Port car plant has said it will place its £8m car fleet order with a rival firm if jobs go.
The Vauxhall Astra is one of Britain's best selling cars
Amicus wants reassurances from parent company General Motors about the future of the Cheshire-based plant, where 1,000 jobs are reportedly at risk.
Amicus is threatening to hold contract talks with Japanese manufacturers.
It may also encourage its one million members, their families, and members of other unions, not to buy Vauxhall cars.
Reports have suggested that General Motors (GM) could cut production of the Astra model and shed a third of the factory's 3,000 jobs.
Last week the carmaker said it was reviewing production capacity across its three Astra plants in Europe as it expects demand for the car to decline.
And remarks by GM Europe boss Carl-Peter Forster raised fears UK job cuts are in the pipeline, when he was reported to have said spreading redundancies across its European plants was "not super attractive".
GM Europe is seeking to cut 12,000 jobs as part of its restructuring and cost-cutting plans.
But Amicus General Secretary, Derek Simpson said: "Unless GM are prepared to treat decent men and women in Britain with some dignity we will cancel our £8m ($15m) 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."
Mr Simpson will meet MPs at the House of Commons on Tuesday to present "incontrovertible proof" that weak UK labour laws are leading to the demise of the UK car manufacturing industry.
Analysts are now saying that dropping one of the plant's three shifts was increasingly likely.
Professor Garel Rhys, at the Centre for Automotive Industry Research at Cardiff University on Monday's Radio 4's Today Programme said he thought it was a "done deal."
The union says that labour laws in the UK allow employers to fire workers with more ease than their European counterparts, and that this was behind recent job losses at Peugeot, Ryton, TVR "and now potentially Vauxhall".
Amicus also says that the Vauxhall Astra was the UK's number one best-selling new car in April "and sold 25,171 cars, a massive 3,072 more than Ford, due to the hard work and commitment of its workers".
The most important next step for the Ellesmere Port is to make sure it gets the next model of Astra according to Professor Rhys.
"It needs to show it is an efficient home for this model," he said, adding that GM has already made a "huge investment" in the plant.
Meanwhile, the union says one million manufacturing jobs have been lost in the UK since 1997, and those who are made redundant have struggled to find comparable employment.