DHL workers employed at Jaguar Land Rover's Halewood factory and plants in the West Midlands are preparing to vote on whether to strike over "serious safety concerns".
Unite is set to ballot its members who are employed by DHL on a logistics, distribution and transport contract for JLR.
The union said DHL has not responded to to workers' concerns about being forced to take defective vehicles onto the road.
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Unite general secretary Sharon Graham said: "DHL is guilty of what amounts to union busting. Not only are workers being placed in danger but Unite reps are being victimised when trying to protect our members.
"DHL should take note that unless these matters are quickly resolved then Unite will leave no stone unturned to secure safe work terms for our members and justice for those who’ve been victimised."
Unite said that if strike action goes ahead at DHL it will cause "severe disruption and likely lead to the plants grinding to a halt".
Unite national officer Matt Draper added: "DHL's attitude is shameful and it is ignoring its own safety procedures, placing workers at risk.
"Unite has been trying for weeks to get these matters resolved without success. They are more interested in attacking stewards and ignoring safety concerns. Unless DHL takes the union’s concerns seriously, a ballot for strike action will begin shortly."
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