A warning by Fianna Fail's Barry Cowen that it would not allow the election of a new Fine Gael leader as Taoiseach amid the row over water charges has been branded "childish".
Fine Gael water committee member Colm Brophy made the remark as tensions between the two main parties escalated over the weekend.
The Dublin South-west TD said he still hopes the committee can reach an agreement this week.
However, he asked if Mr Cowen would write to Fianna Fail members saying "he won't give permission for Fine Gael members to have Easter eggs either".
"That's the level of ridiculousness of sending out letters like that," he said.
However, Mr Cowen was bullish in the face of the criticism, saying Fine Gael TDs will not have any "worries" if their party legislates for the wishes of the water committee.
Fine Gael insists that the committee's current draft report will not comply with a "polluter pays" principle under EU law.
The committee has until the Easter recess to produce a report to be voted on in the Dail.
The Sunday Independent revealed that Mr Cowen wrote to Fianna Fail members telling them that the party will not facilitate a new Fine Gael leader becoming Taoiseach unless the confidence and supply deal between the parties is honoured on water charges.
In the email, Mr Cowen insisted Fianna Fail's policy on water is "financially, environmentally and legally sound".
Mr Cowen told the Herald that Fine Gael "need not have any worries if they simply commit to bring forward legislation to give effect to the wishes of the committee and the Dail".
He reiterated his view that failure to do so "undermines" the confidence and supply arrangement.