Fine Gael has pledged to reverse the controversial tax plan to favour two-income households if re-elected to government, dramatically increasing the already intense pressure on the Government on the issue.
As Fianna Fail deputies face constituency anger over the proposal this weekend, the Fine Gael leader, Mr John Bruton, also warned employers and trade unions there were "risks" involved in their agreeing a new social partnership deal on the basis of tax plans that Fine Gael would change.
Pressure on the Government to modify the effect of the Budget is expected to intensify next week when Fianna Fail deputies return to Leinster House after a weekend of hearing constituents' views around the State. However the Government is so far resisting backbench pressure, saying there will be no change.
Mr Bruton's pledge to reverse the plan has raised the prospect of this becoming an issue upon which Fine Gael can campaign from now until the next general election. It has also injected great uncertainty into the tax reform process, as it threatens a substantial reversal in policy direction if the Government changes.
A Government spokesman insisted yesterday there would be no amendment to the proposal to "individualise" tax bands over the next three years, thus giving substantial tax advantages to dual-income couples over single-income couples. This is a central plank of Mr McCreevy's Budget, Government sources say, and to change it would involve rewriting the entire taxation package.
The Government received support for its plan yesterday from the Combat Poverty Agency, which said the "individualisation" controversy only affected "the well-off tax-paying households". Money saved through the measure should be used to increase child benefit, the agency said in a statement.
SIPTU's national equality secretary, Ms Rosheen Callender, also welcomed the change in principle, but said there were better ways to recognise the additional work-related costs that arose where two parents in a family were working.
However, faced with unprecedented public opposition, Fianna Fail deputies are pressing for extra measures to benefit single-income families to offset the effects of Mr McCreevy's change.
In an attempt to meet these demands, Mr McCreevy pledged last Thursday that the Fianna Fail manifesto commitment to introduce a £2,000 benefit to recognise the work done by women in the home would be met during the lifetime of the Government.
This would only partly offset the effect of the "individualisation", which when fully implemented after three years would benefit dual-income families currently on top-rate tax by more than £6,000 a year. In addition, backbenchers want action now, rather than in the next Budget, to ease the pressure they are feeling from constituents.
Officials are understood to have been asked to examine measures that would offset the changes, but a Government spokesman said last night there was no intention "as of now" to introduce any such measure in the Finance Bill to be published in the new year.
However, the pressure for immediate change increased yesterday, with demands for a rethink from the Archbishop of Dublin, Dr Desmond Connell, and the Irish Family Planning Association pointing up the breadth of the opposition to the plan.
Fianna Fail TDs report receiving an unprecedented number of phone calls complaining about the change, and are certain to face further protest at constituency clinics throughout the State this weekend.
Ms Mary Hanafin, one of the Fianna Fail TDs who met Mr McCreevy yesterday, called on him to introduce measures to benefit single-income families immediately. Mr Bruton said last night the consensus between parties that allowed major taxation change to take place over a period of years had now ended. "I will do everything I can to reverse the unfair and divisive change in the tax system contained in this Budget."