Former MEP Patricia McKenna is to quit the Green Party and run in the forthcoming European elections as an Independent candidate.
Ms McKenna said she took the decision to leave the party because of its performance in government.
In an interview with
Hot Pressmagazine, to be published tomorrow, she brands her Green Party colleagues in the Dáil as "nothing but hypocrites".
"I feel embarrassed about being a member of the Green Party because of what we said in the past and the promises we made, which we failed to deliver on," she said. "I just knew that I couldn't run under a Green Party ticket and pretend that everything was alright because I'd be lying."
She accused the party of "selling out" and said they would probably "be glad to get rid of me because I keep reminding them of the promises they have reneged on".
She claimed Minister for the Environment John Gormley has lost all credibility and accused Minister for Communications Eamon Ryan of abandoning key Green campaigns. "I really don't know how Eamon can live with himself after all his promises," she said.
Senator Deirdre de Burca, who is the party's Dublin candidate in the European election, said she was not surprised at the decision and claimed Ms McKenna offered the Greens nothing but criticism.
"The pity about Patricia McKenna's move is that she did not do it at least a year ago and spare herself and her party colleagues a lot of time-wasting and strife," the Senator said.
"In my view she is addicted to the politics of opposition and appears reluctant to move beyond that comfort zone," she said. "The Green Party is committed to the constructive politics of engagement and bringing about real change from within."
Ms McKenna became Ireland's first Green Party MEP in 1994 when she was elected in Dublin. She lost the seat in the 2004 election.
She failed to be elected in the Dublin Central constituency in the last general election and lost out to Mr Gormley by a two-to-one majority in a vote to replace Trevor Sargent as party leader in July 2007.
She strongly opposed going into Government with Fianna Fáil and campaigned against the Lisbon Treaty in last year's referendum, which was supported by the party's leadership.