Members of Amicus this week voted by 340 to 310 in favour of a strike - a decision in line with the results of a similar vote by TGWU members.
Peugeot has offered a deal worth 7.3% but unions want the offer improved.
And Nissan could be facing its first strike at the Sunderland plant – again over a pay dispute. The company is believed to be set to present a two-year pay offer, worth 3% in each year, to the 4,672 employees.
Amicus, which claims to have been excluded from the pay talks, says that the Sunderland plant is "dangerously close" to a stoppage.
A Nissan spokesman countered by saying that all shop stewards at the plant were members of the company's works council - and it was up to that body to invite a full-time official from Amicus to any talks.