The Association of Secondary Teachers in Ireland has rejected the Government's proposals to restore pay equality for so-called new entrants by 53% to 47%.
Members were balloted today. The turnout was 58%.
The ASTI Central Executive Council made no recommendation to members on how they should vote.
This brings the ASTI into line with the Irish National Teachers Organisation, which rejected the proposals by 53% to 47%.
However, their sister union the Teachers Union of Ireland accepted the proposals by 53% to 47%.
Under the proposals published in September, more than 60,000 public servants who have been on lower pay rates since 2011 are set to receive increases averaging more than €3,000.
Commenting on the result of the ballot, ASTI President Breda Lynch said members are firmly committed to achieving equal pay for their lower paid colleagues.
She said they would remain steadfast in their determination to achieve justice for their lower paid colleagues in terms of equal pay for equal work.
She noted that in 2016 the ASTI took strike action over the discriminatory pay of post 2010 teachers - which had led to them losing pay and suffering other financial penalties under emergency legislation introduced during the economic crisis.
Ms Lynch said that that industrial action had put the issue of pay inequality firmly on the agenda, so that the government could no longer ignore it.
She said the current government proposals represent insufficient progress, and do not achieve equal pay.
Ms Lynch said the ASTI would continue to pursue the full restoration of pay for lower paid colleagues.
The executive will meet shortly to consider the outcome, and will probably hold talks with fellow teacher unions before proceeding to any further steps.
A spokesperson for Minister for Education and Skills Joe McHugh said that he noted the result.
In a statement, they said: "It is of course a disappointment that a majority of members of a second union did not back the new pay deal.
"But it should be noted that the ASTI and INTO have operated the terms of the public pay deal and everyone in Government hopes that they will continue to do so.
"It's accepted that outstanding pay differences for teachers who qualified since 2011 are a significant issue.
"But this Government is making gradual improvements on that front and the latest measures, announced on 24 September 2018, deliver more considerable improvements for teachers and will continue to do so."
In a statement, the Department of Public Expenditure and Reform said that Minister Paschal Donohoe is disappointed with the result of the ballot.
The statement said "As he [the minister] has said on numerous occasions only those inside the Agreement can expect to enjoy the benefits of the Agreement."