Taoiseach Brian Cowen is hitting out at Fine Gael's Simon Coveney after he tweeted that Mr Cowen sounded between drunk and hungover on a radio interview this morning.
Mr Cowen this afternoon described it as a real new low in Irish politics.
He said the Fine Gael deputy made "an appalling and unfounded assertion".
Mr Cowen added he has a hoarseness for which he apologises and that the Opposition's claims were "politically motivated and orchestrated".
The row follows an interview by Mr Cowen with RTÉ Radio's Morning Ireland in which he mistakenly referred to the Croke Park Agreement on public sector pay as the Good Friday Agreement.
Deputy Coveney, Fine Gael's transport spokesman, this morning tweeted: “God, what an uninspiring interview by Taoiseach this morning. He sounded half way between drunk and hungover and totally disinterested.”
"I've very sorry that Deputy Coveney would resort to that type of petty, personality-type politics which I find distasteful and simply, totally uncalled-for," the Taoiseach said this afternoon.
"As I say, I apologise for my voice and the articulation of the points I was making but the substance of what I had to say I stand over."
Mr Cowen's ministerial colleagues have admitted that the Taoiseach had a late night following the opening of Fianna Fáil's Parliamentary Party conference in Galway, but have jumped to his defence saying the Opposition charges are out of line and that there was no issue with the substance of what the Taoiseach said.
Foreign Affairs Minister Micheál Martin criticised Deputy Coveney, saying: “I would have expected better from Simon.”
Speaking to Cork's Red FM today Deputy Coveney defended his comments.
"The country needs a Taoiseach at the moment who is inspiring people, who has ideas, who has drive, who has energy" the Cork TD said.
"Anyone listening to that interview this morning would have got the opposite impression."