Workers from AB Connectors at Abercynon in South Wales began a continuous overtime ban on Tuesday of this week over a pay freeze.
Some 123 workers in the Unite union at the company, which is part of the TT group, have rejected its decision to freeze their pay.
AB Connectors told staff that all other workers across the group had been asked to do the same.
But workers in parts of the group received a 2.85 percent pay increase. Others are also preparing to ballot for action.