Tesco is calling on Mandate to re-think its strike after six stores voted against industrial action.
The union began balloting members from another 23 stores last night after the company rejected the proposals put forward by the union on Friday.
The ballot will continue for the next two evenings - four stores out of the ten that voted last night opted to join the picket.
Tesco says there is an onus on workers to accept a Labour Court recommendation and is accusing the union of trying to undermine the court.
Mandate general secretary John Douglas says they've already rejected that recommendation and they're not going to be forced back to work.
"Our members are rock solid on the picket line, so it's only going to go one way and unfortunately, not of our making, there'll be more stores joining the strike over the next week or so until it's resolved," he said.
"There's actually no need for this at all. If the company just accepted the position that we put to them last Friday the pickets could have been lifted."