Sligo-Leitrim TD Marc MacSharry has launched a stinging criticism of the Taoiseach as he confirmed he is leaving Fianna Fáil.
In a statement, Mr MacSharry claimed Micheál Martin failed to allow proper investigation into a claim of bullying made against him by a local councillor.
The claim was made by Sligo councillor Donal Gilroy over a message MacSharry posted in a party WhatsApp group.
The now Independent TD alleges the Taoiseach used the complaint for political gain at national level to delay and prevent his re-admission to Fianna Fáil, which had been expected in recent weeks.
Mr MacSharry originally resigned the party whip over the Katherine Zappone controversy last year.
In the statement, which he posted on Twitter, Mr MacSharry said "this recent complaint related to exchanges between us over several months on services I have secured for Sligo University Hospital and their significance".
He said it was a "a robust issue and robust language was used".
"Instead of arranging an investigation of this local political dispute, the Taoiseach used the complaint for political gain at national level to delay and prevent my re-admission to the parliamentary party.
"He knows that I am critical of his performance as Taoiseach and his support within the parliamentary party is wafer thin."
Mr MacSharry claimed his re-admission "would have been a problem for him, so he has used this complaint for his own purposes".
"The informal investigation promoted by the Taoiseach is a non-starter as the confidentiality required has been broken by the public statements of the Taoiseach and the Chief Whip.
"The Taoiseach has tried to brush this under the carpet with claims that I have not engaged. This is despite numerous correspondence from I to him, the Chief Whip and party officials on the matter setting out my position clearly."
Mr MacSharry also included Mr Gilroy's complaint in the social media post.
He said he included it "to show the highly politically motivated nature of it and its unquestionably suspicious timing on the eve of the Fianna Fáil parliamentary party meeting for my re-admission".
He called the complaint "an unwarranted attack on the contribution of me and my family to political life with over 40 years' service in the Oireachtas".
In response to the matter, a spokesperson for the Taoiseach said: "We have received Marc MacSharry TD's resignation from Fianna Fáil. We wish him well for the future.
"We strongly reject the assertions made by Deputy MacSharry in relation to the written complaint received by the Party."