Some 600 workers in the GMB union at Bakkavor Pizza Harrow are balloting for strikes over pay.
Workers have suffered below-inflation pay deals for the last four years.
They supply pizzas to major supermarkets.
Workers rejected a 2.5 percent increase and a £60 Christmas bonus in November.
Bosses then offered a £10 voucher to be used in the staff shop earlier this month.
The ballot result will be known on 29 January.