Independent TD Stephen Donnelly said he is quitting the Oireachtas Banking Inquiry as the actions of Taoiseach Enda Kenny have made it no longer credible.
Mr Donnelly said that due process had been followed in the Seanad but that Mr Kenny, in his actions and words, had undermined the inquiry.
Last Thursday, Fine Gael Senator Michael D'Arcy and Labour Senator Susan O'Keeffe were added to the committee amid scenes of loud protest in the Seanad.
Mr Donnelly said the Seanad select committee had made its decision to appoint senators Sean Barrett and Marc McSharry and that should have been that.
He said that Mr Kenny had made a very politicised attack on Fianna Fáil in the Dáil and had subverted the will of the Oireachtas by ensuring a Government majority on the inquiry.
Meanwhile, Taoiseach Enda Kenny and the Tánaiste Eamon Gilmore have spoken to each other this afternoon about the banking inquiry in the wake of the resignation of Independent TD Stephen Donnelly.
A spokesperson for the Government said that both confirmed that no whips would be imposed on party members serving on the banking inquiry.
The spokesperson said that this is a fundamentally unique inquiry in its nature and said members of Fine Gael and Labour who are on the committee would not be subject to a whip.
Minister for Transport Leo Varadkar said it was always intended that the inquiry would reflect the balance in the Dáil and Seanad.
He said Fianna Fáil had grabbed two seats for themselves in the Seanad and asked what the controversy might be like had Fine Gael and Labour taken advantage in the same way.
Mr Varadkar said, speaking as a citizen, he was let down to hear that Mr Donnelly was standing down from the inquiry.
He said Mr Donnelly was exactly the type of person who should be on the inquiry and he was elected to do this type of work.