Fianna Fáil TD Marc MacSharry has resigned from the parliamentary party and voted no confidence in Minister for Foreign Affairs Simon Coveney.
A spokesperson for the Taoiseach and Fianna Fáil leader, Micheál Martin, said: "The Taoiseach regrets the resignation and wishes Marc well."
In his letter of resignation, Mr MacSharry said: "I was elected to serve a democratic republic - not one which applies different rules and sanctions depending on the identity or the position of people involved."
Yesterday, the TD for Sligo, Leitrim, South Donegal and North Roscommon, formally requested that his parliamentary party meet no later than 4pm to consider how TDs and Senators would approach the vote, but the meeting was never held.
In a letter seen by RTÉ News, Mr MacSharry had said: "As democrats I am certain we all agree that we take a collective position rather than one decided for us or dictated by the one or the few without proper consideration by all members."
In his resignation letter, he also said: "All Fianna Fáil party positioning and policy are being determined in a fashion consistent with an undemocratic totalitarian regime rather than that of a democratic socialist republican party of and for the people."
He added: "I therefore have no option but to resign the Fianna Fáil parliamentary whip effectively immediately and will be voting no confidence in Minister Coveney this evening."
Mr MacSharry has said he will continue to work as a TD for his constituents, adding: "I will work to the very best of my ability in line with my Fianna Fáil traditions, its constitution, and values from which the current leadership regime have unilaterally chosen to completely depart."
Minister of State for Business, Damien English, said it was "regrettable" that Mr MacSharry had chosen to resign, but he was not surprised.
He told RTÉ's Drivetime he had not seen Mr McSharry's letter but his reasons for quitting were a "matter for him and the Taoiseach".
Mr MacSharry has long been an outspoken critic of Mr Martin, arguing the party was being damaged by his leadership.
Additional reporting Mícheál Lehane