More than 700 Co-operative Food Group drivers are being balloted for industrial action over a plan to transfer around 50 drivers in the Midlands to haulier Eddie Stobart this April.
Unite, which represents the drivers, said the ballot had been prompted by fears the group will outsource other transport jobs across the group in the future.
The ballot, for strike action or industrial action short of a strike, will be held from Tuesday 1 March until Monday 14 March.
The Co-op driver members being balloted are based at Co-op depots at Birtley, Newcastle; Carrickfergus, Northern Ireland; Castlewood, north Derbyshire; Coventry; Inverness; Newhouse, Lanarkshire; Plymouth and Thurrock in Essex.
Andy Perry, logistics director at Co-operative Food, has previously written to Co-op transport workers assuring them that it has no plans to outsource the other areas of transport it operates, or any of its warehouse functions.
Perry has said the decision to outsource the sites has been driven by the cost of empty running, and the change would allow it to re-invest and improve its core depot to store operations.
However, Unite national officer for retail distribution Adrian Jones said: “Basically we have heard it all before and while it may be copper bottomed, it is not cast iron. Our members are not yet convinced that this will not happen again.
“Our members want to stay with the Co-op and our members nationally are intent on supporting their colleagues in Coventry.”
Co-op revealed last month that it was outsourcing its trunking operations at its national DC in Coventry, as well as its primary operations at Castlewood to Buffaload.
Responding to today's announcement by Unite, the Co-op said: “We are aware that Unite has called a ballot regarding a proposal to outsource elements of our trunking operations.
We have been engaged in a lengthy period of discussion with all of our recognised trade unions, and we will continue to support our colleagues through this process and engage with trade unions to allay their concerns.”