More than 600,000 Irish workers could get a "nasty shock" when they look at their payslips in January, according to Fianna Fáil.
Party spokesperson on finance, Michael McGrath, has warned that the allowances of low and middle income workers could be the worst hit following a review by the Revenue Commissioners of flat-rate expenses.
Citing the Revenue website, Mr McGrath said that the outcome of any changes will likely come into effect in January 2020 and warned that it would be a "really significant issue for a lot of workers" across various sectors.
The Cork South-Central TD said that it involved expenses of over €160 million annually with €48 million in tax saved.
Mr McGrath cited figures such as the €1,312 allowance that workers from Tara Mines had, with a saving of more than €500, while many nurses had an allowance of €733 with a tax saving of almost €300 annually.
Mr McGrath told the Dáil: "We are getting close to the deadline and we don’t know where this is going.
"There could be a nasty shock in January for tens of thousands of workers if not more when they see their pay slip.
"We should be made aware of them and be given an opportunity to debate them in the House and to discuss with the Revenue in the finance committee the natures and scope of these changes and to see if any change is possible."
Minister for Finance Paschal Donohoe admitted that he was "well aware of the number of citizens affected" but insisted that the administration of the tax code was a matter for Revenue and revealed how he expected them to implement the outcome in their "customary proportionate and fair manner."