Nissan car workers in Sunderland rejected a 2 percent pay offer for the second time last Sunday.
More staff also joined the Amicus union. The Sunderland plant could be facing the first strike in its 20-year history.
A vote held last month rejected the two year offer by 51 percent to 49 percent. Amicus represents 1,500 of the 4,500 workforce.
At a meeting on Sunday with national and regional union officials, members backed plans to reject Nissan's pay offer.
Regional Amicus organiser Dave Telford said, 'The meeting went very well and we got a dozen new members out of it.
'Non-members heard what we had to say and obviously like it as they signed up there and then.
'We reaffirmed our commitment to ballot on industrial action in event of the pay offer being voted down again.'
He said that members felt the pay offer would be well below the cost of living and inflation, and the union would continue to increase its membership to make any industrial action as strong as possible.
Telford said, 'Amicus membership at the plant is about 30 percent and we are not putting a figure on how much we want to increase that,' he said. 'The recruitment campaign is going well.'