The Ford Motor Company is set halve its production of the Transit van at its plant in Southampton.
The Unite union, which was briefed by managers on Monday, fears the plans could lead to the closure of the Hampshire plant in the long term.
Ford said it was planning to continue making the Transit panel van at the Swaythling plant until 2011.
After that it wants to use the site to make only a specialist version of the model called "a chassis cab vehicle".
The plant currently produces 75,000 transit vans a year but this will be reduced to 35,000 a year.
Discussions with Unite are continuing and no final decision has been taken on the plans.
About 1,100 people are employed at the city plant and Ford warned cuts in production could lead to job losses.
The move follows the termination of 124 short-term contracts at the plant last month.
Reduction of "production volumes" there began on 26 August.
Unite said it was opposed to the proposals, which it believed would lead to the closure of the Swaythling plant in the long term.
Unite's national officer Dave Osborne said: "Unless Southampton and the future of other Ford UK plants are secured, there will be no recommendation for settlement in the forthcoming pay negotiations.
"Securing investment and the future of our members' jobs is of paramount importance for Unite.
"Failure to agree such security will create a major stumbling block.
"Ford's proposals are a direct contradiction to the guarantees they have made to the UK and to the practice of locating where they sell."
Brian Bennett, from Ford, said: "Ford has outlined a multi-million pound investment plan for Southampton as primary source for chassis cab variants of the next model Transit.
"This financially viable Ford Transit investment plan and the associated impact on employment levels will continue to be reviewed.
"The European new Transit programme will be approved at board level in the US in 2009.
"The market for cars and commercial vehicles is intense and it is vital that Ford vehicles and locations remain cost competitive."
Ford Transit production moved to Southampton in 1971 and the five millionth vehicle was made in 2005.
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