THE largest public sector union has warned members that sustained industrial action is their only option if the Government imposes pay cuts.
IMPACT also told its 63,500 members it could not launch a successful legal challenge if their wages were slashed.
In a message explaining why they should vote 'yes' to the new Croke Park deal, the union said the Government could force deeper pay cuts on more staff if they did not back it.
In contrast, it said the chief union negotiators had managed to ensure that 87pc of them would not suffer wage reductions.
Those earning over €65,000 a year face pay cuts of 5.5pc and above.
The union said the agreement is "the most challenging proposition put to public servants in living memory" but the best package that could be achieved.
This was because it was not possible to make the Government's determination to reduce the pay bill by €1bn over three years "go away".
"If the proposals are rejected, IMPACT believes it would not be possible to achieve a better negotiated outcome," it said.
"Equally, the union has said that any attempt to impose cuts and changes to working conditions without agreement would be a breach of the existing Croke Park agreement. The union believes that any effective resistance to this would require sustained industrial action."
It said the issue was whether IMPACT members believed it was possible to achieve a better deal if the current one was rejected.
"In our judgment, it isn't," the union said, adding that legal advice from the last round of pay cuts in 2010 said their contracts were "as determined or sanctioned by the relevant minister from time to time".
"On this basis, the union does not believe a successful legal challenge can be mounted against imposed pay cuts," it said.
Meanwhile, divisions over the agreement among firefighters have deepened.
Fire station officers in Dublin have been ordered to take down posters by a union opposing the deal.
A memo from Assistant Chief Fire Officer Michael O'Reilly said station notice boards were being used by "non-recognised associations".
He asked station officers to ensure only notices by "recognised unions", and not the Irish Fire and Emergency Services Association (IFESA), were posted on the boards.
Members of IFESA claimed Mr O'Reilly is a member of SIPTU, which stayed at talks and brokered a deal that protects premium pay from cuts.
IFESA is a breakaway group from SIPTU and is opposing this deal on the basis that other frontline workers would still face cuts.
Spokesman Richard Kelly said the group's chairman John Kidd, who was previously barred from the fire stations, was a "modern-day Jim Larkin".