More than 200 workers at the Marks & Spencer distribution centre in Swindon struck on 22 December over plans to introduce worse conditions and lower pay for new starters.
The workers are in the Unite union, and are employed by distribution and warehouse firm Wincanton.
Unite is also balloting 123 oil tanker drivers on Wincanton’s ConocoPhilips contract.
The company wants to move drivers onto new contracts with worse conditions and a 20 percent pay cut.
Unite national officer Matt Draper said, “We have negotiated in good faith but Wincanton is bringing us closer and closer to casualisation of this industry.”
The ballot is due to close on Wednesday of next week.