The Irish Federation of University Teachers has said it is to commission legal experts to examine the legislation allowing the Government to cut public sector workers' pay without agreement.
The union says the proposed legislation will treat non-union staff as if they had voted against the Haddington Road Agreement and force them into taking pay cuts.
Speaking on RTÉ's Morning Ireland, IFUT General Secretary Mike Jennings said that the federation is commissioning a senior counsel to review the legislation ahead of a special consultative conference on 8 June.
After this the executive will decide on whether to recommend acceptance or rejection of the new offer.
"We believe that there are constitutional implications for the legislation because to our knowledge it is the first time that the treatment of employees, including the salary of employees, will be dictated to by their membership or non-membership of a union, rather than the work that they do and the grade that they hold."
The Haddington Road Agreement aims to reduce the public pay bill by €1bn by 2015.
It does not, however, include any specific guarantee that the pay bill for 2013 will be reduced by €300m as the Government had wished.