The Department of Public Expenditure and Reform has instructed all Government departments to commence moves to cut over 80 allowances for serving state employees.
Up to now, the Government has primarily targeted new entrants for cuts in allowances - but now current employees could face reductions in their income from allowances.
SIPTU pledged to resist the move, saying that allowances were protected under the Croke Park Agreement.
Last month, Minster Brendan Howlin announced that following a review of around 1,100 public service allowances, only one was being abolished for existing staff - though some were eliminated for new recruits.
However, in a letter to each Government department, the Secretary General of the Department of Public Expenditure and Reform Robert Watt outlines a priority list of allowances to be abolished or reviewed for serving government employees.
He urges immediate engagement with staff representatives to secure agreement to the elimination of those allowances payable where no business case exists to pay them to new entrants.
Some could be bought out with a one-off lump sum worth one and a half times the annual loss.
In other instances, access to the allowance could be modified.
Both mechanisms could lead to savings in the pay bill.
The Departmental reviews must be completed by 28 February.
However, SIPTU official Paul Bell warned that any attempt to cut allowances - which they view as core pay - would put them on a collision course with the government.