The European Commissioner for the Environment has expressed doubts that a proposal to impose fines on those wasting water would comply with EU law.
Commissioner Karmenu Vella said it was questionable if such a move would bring Ireland in line with the EU Water Framework Directive.
A majority of TDs in the Dáil are opposed to water charges.
The special Oireachtas committee has until 14 April to agree a motion that will then be put to a Dáil vote.
This Tuesday the committee will hear legal opinion on a Fianna Fáil idea to use existing legislation to fine those wasting water.
It was viewed by many as the basis for an agreement between Fine Gael and Fianna Fáil to effectively end domestic water changes.
But now the European Commission has expressed major doubts about this proposal in a letter seen by RTÉ's The Week in Politics.
The letter to Fine Gael MEP Brian Hayes from Mr Vella says the Fianna Fáil proposal would not establish a reliable source of revenue to cover the cost of water services.
The commissioner says too that Ireland cannot revert to previous practice, that is, paying for water through general taxation.
Mr Hayes believes this means some form of water charges based on the polluter pays principle are required.
The MEP called on the European Commission to now set out unequivocally its legal view on water charges to the Oireachtas Committee.
Sinn Féin's spokesperson on Housing, Planning & Local Government Eoin Ó Broin described the contents of the letter as "extremely revealing", adding, it "confirms that Simon Coveney's charged for a so-called excess water use is a Trojan horse for universal water charges".
Solidarity/PBP's Paul Murphy said the commission's "intervention" falls into the "well they would say that, wouldn't they?" category.
He said while Fine Gael will probably use Mr Vella's letter to "raise the spectre of massive fines to insist that water charges continue, the facts don't back up their position".
