Unite is to consider temporarily withdrawing political and financial support from the Labour Party, until it receives assurances that commitments to protect workers' rights will be met.
Delegates at the union's conference in Belfast passed a motion which accused Labour ministers of openly promoting the downgrading of workers rights, pay and conditions of employment.
Delegates also passed a motion calling on Labour to withdraw from the Fine Gael-led Government.
However, a motion calling on conference to cease any financial contributions to Labour from the union's political fund was withdrawn.
Earlier, the union pledged to support its public sector members in "whatever action they deem necessary" to protect members' interests in opposing the Haddington Road proposals on public sector pay.
Unite's 6,000 public sector members voted overwhelmingly to reject the deal in a recent ballot.
Representatives from Northern Ireland and the Republic attending the union's Irish Policy Conference in Belfast voted overwhelmingly for an emergency motion supporting state employees.
The motion describes the Haddington Road proposals as unacceptable.
Unite's public sector representatives will meet on Friday to consider the result of other unions' ballots on the deal.
So far, Unite is the only union to reject the proposals in a ballot.
The Government has consistently warned that any union that rejects the Haddington Road deal will lose the protection from even harsher pay cuts and compulsory redundancy once emergency legislation comes into force on 1 July.
Regional Secretary Jimmy Kelly criticised the Government for introducing the legislation and disputed whether the deal would save €1bn.
He also warned that the same trend would spread to private sector workers.
Mr Kelly said Unite would support 100% whatever their public service members decided.
Delegates accused the government of putting a gun to the heads of workers and described the changes sought as draconian.
Mick McKenna said that in his workplace, Enterprise Ireland, an email had been sent out last Friday stating that from 1 July wages would be cut by a minimum of 5.5%, the net effect of which would be to bring pay back to 2005 levels.
Another delegate said that he had lost €1,000 a month and was likely to lose more.
There was also criticism of the leadership of the Irish Congress of Trade Unions.
Unite represents around 1.5 million workers in Britain and Ireland.