The potential for public sector pay hikes will be wiped out unless State employees agree to continue working 15 million 'free' hours every year.
Public Expenditure Minister Paschal Donohoe is set to warn unions that the so-called 'Croke Park Hours' must be considered the new norm if they want a pay deal.
Analysis carried out by the Department of Public Expenditure indicated the value of the additional hours currently being worked at "somewhere in the region of €600m per year", the Sunday Independent can reveal.
And according to the minister he would have to hire an additional 12,000 staff to just maintain frontline services if the hours are ended.
"The loss of those hours would have a significant budgetary effect and a significant effect on the delivery of frontline services.
"The productivity changes secured as part of those agreements have been vital in allowing the maintenance or improvement of public services," Mr Donohoe said.
Under the current arrangement, public servants such as teachers and nurses work just over two extra hours a week, or 27 minutes a day, bringing their working day to seven hours, 24 minutes.
Mr Donohoe's comments are the first clear indication of a 'red line issue' emerging in advance of the talks.
One of the unions' top priorities is to have the unpaid hours abolished. Delegates at the Civil, Public and Services Union's annual conference earlier last month backed a motion to ballot on strike action if the Government refuses to roll back the hours by June.
However, Mr Donohoe argued the hours are used to cover absences in the health service, avoid schools closing for parent-teacher meetings and to extend opening hours in libraries.
He told the Sunday Independent the talks will be "incredibly difficult" but he has a "clear understanding" of what an affordable agreement will look like.
"We know what we need to do. We know what the investment needs are in transport, in hospitals and in schools.
"We know what we need to do in terms of public pay to allow us deliver that balancing act," he said.
Mr Donohoe refused to speculate on what level of pay rise workers might expect between 2018 and 2021 but warned next year will be particularly challenging for the exchequer due to social welfare and childcare promises made by the Government.
"One of the reasons why I talk about an extension to the Lansdowne Road Agreement is the litmus test of any change cannot be 'is it affordable in the year that we give it?'. It has to be 'is it affordable in the years afterwards?'. That's why I believe an extension of the Lansdowne Road Agreement is the way to go," he said.
The public services committee of the Irish Congress of Trade Unions has indicated that negotiations on pay should begin sooner rather than later. "Our priority is to restore incomes quickly."