The Association of Secondary Teachers in Ireland has voted to reject the latest public service pay agreement "Building Momentum" because it does not restore full pay equality for staff recruited since 2011.
Members rejected the new deal by a margin of 79% to 21%.
The two-year successor to the Public Service Stability Agreement covers around 350,000 civil and public servants, and will add around €900m per year to the public service pay bill when fully implemented.
The ASTI is the first public service union to deliver a ballot result on the deal, and while it is a rejection, the deal is expected to be carried on an aggregate basis because the largest unions including Fórsa, SIPTU, the Irish Nurses and Midwives Organisation and the Irish National Teachers Organisation have recommended a yes vote.
If accepted, Building Momentum guarantees two 1% pay rises in October of 2021 and 2022, along with potentially a further 1% through a process of sectoral bargaining.
There will also be a process leading to the rolling back of unpopular additional unpaid hours imposed on State employees in the 2013 Haddington Road Agreement, and the restoration of pre-austerity rates of overtime and premium payments.
However, despite those benefits, the ASTI membership has voted to reject the deal on the basis that so-called "new entrants" are still suffering pay disadvantage.
Some have now been working in the public service for over ten years on pay scales 10% lower than their pre-2011 colleagues, as well as losing out following the abolition of certain allowances retained by their older counterparts.
While some interim measures involving increments have reduced the gap, the ASTI says members remain at a disadvantage.
President of the ASTI Ann Piggott said the union was sending the Government the same message it had been sending for a decade; that lesser paid teachers must be receiving the same pay for doing identical work.
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She said the union had calculated that a newer entrant would lose up to €29,000 in the first five years of their career compared to a pre-2011 colleague - with that loss rising to around €86,000 over 25 years.
Ms Piggott said the "Building Momentum" agreement did not fill that gap.
She noted that many teachers at the beginning of their careers did not have a full time job, but rather a few teaching hours per week.
"Six to seven hours on a lesser pay scale is not enough for them to live on and as a result they are choosing to go abroad," the ASTI President said.
Asked whether a 28% turnout was sufficient to warrant rejection of the deal that could give members a 3% pay rise over two years, Ms Piggott said that was the outcome, and everyone had had a chance to vote.
She said the result itself - a 79% rejection - was significant.
She said the ASTI would be part of the aggregate vote of public service unions on Building Momentum and the outcome of the overall vote would be considered by the union's executive.
Asked whether there was a possibility that the ASTI would adopt a unilateral position, she said that would be up to the executive.
So far, only three unions have recommended rejection of Building Momentum.
Both the ASTI and the Teachers Union of Ireland urged members to vote no because the deal did not restore pay equality for post-primary teachers - though their primary school colleagues in the INTO have backed a yes vote.
The Medical Laboratory Scientists Association, which represents around 1,800 scientists, also recommended rejection because a long-running pay inequality grievance has not been resolved.
The outcome of the public service union ballots will emerge over the coming days, and the deal is expected to be formally ratified at a meeting of the Public Service Committee of the Irish Congress of Trade Unions on 23 February.
A spokesperson for the Department of Education said Minister Norma Foley is "disappointed" the ASTI has rejected the agreement.
"It was a firm objective of the Government to maintain the momentum of reform and change exemplified by the public service during the public health emergency," the statement said.
"The proposed agreement can provide certainty and stability for the Government, public service users and public servants over the coming years".