Bakers at Park Cakes in Oldham and Bolton are to ballot for strikes over pay and conditions.
Bosses are trying to impose worse contracts for new staff, taking away shift payments and overtime rates and cutting sick pay and redundancy.
The 1,000 workers are members of bakers’ union BFAWU. It says the new contracts will create a two-tier workforce.
Park Cakes is owned by private equity firm Vision Capital, worth £2.7 billion.
Across its two sites the workers make cakes, desserts and pastries for shops including Marks and Spencer, Tesco, Sainsbury’s and Asda.