Green Party leader John Gormley has insisted the party is united following the resignation of Déirdre de Búrca from the Seanad.
Speaking in Dublin, he dismissed the criticisms outlined in her letter of resignation as "entirely without foundation".
Mr Gormley said: "We are united as a parliamentary party, we are united as a Green party, and we received an overwhelming endorsement from our party members.
“I believe if our party members were to look at the very swift progress we’ve made since our convention in relation to the implementation of policy they would be even more supportive.”
In a statement this morning, Ms de Búrca announced she had resigned from the Green parliamentary party and Seanad with immediate effect, saying she could not longer continue to support the Greens in Government.
“I have written to my party leader, John Gormley, explaining to him the reasons for my resignation” she said. “I believe that we have lost our way as a party and have gradually abandoned our values and our integrity since becoming part of this Government,” she said.
“I do not take the decision to tender my resignation lightly but am very clear that I cannot continue to be a part of what the Green Party is supporting in Government.”
A spokesman for the Green Party said they were aware of the resignation and would respond in due course. “It is disappointing but not unexpected,” he added.
In her resignation letter, the European affairs spokeswoman tells Mr Gormley, the Greens "in many respects have become no more than an extension of the Fianna Fáil party," adding she had lost confidence in his leadership.
Expressing her belief the party was "paralysed" by the electorate’s rejection of its candidates, including herself, she said: "Any suggestion that we challenge Fianna Fáil, or face it down over important issues, seems to bring up a great fear in us that we will have to leave government. In fact staying in government appears to have become an end in itself now for the Green Party.
"While I was always aware that our political inexperience as a party would leave us vulnerable to being manipulated by Fianna Fáil in Government, what I hadn’t predicted was the strong attachment to office that appears to have developed since we became part of government," Ms de Búrca said.
"It is with regret also that I must also inform you that I have lost confidence in you as party leader."
Accusing Mr Gormley of refusing to take a stronger line with Brian Cowen and Fianna Fáil over core issues, despite being "asked many occasions over the past few months by the Parliamentary Party," she said Mr Gormley had "clearly been unable, or unwilling to do so".
The Senator said this unwillingness to act meant the Greens were "slowly haemhorraging support because of a growing public perception that we have lost the courage of our convictions and have become no more than an obedient ‘add-on’ to Fianna Fáil".
In the letter, Ms de Búrca refers to a report from Dr Niamh Brennan on the issue of the Dublin Docklands Development Authority and said she lacked the confidence the findings of the report "will be acted on in the timely and appropriate manner that the public interest requires".
Recently, there was speculation Ms de Búrca was being considered for a position on incoming EU commissioner Máire Geoghegan-Quinn’s cabinet, but she was not included in Ms Geoghegan-Quinn’s team.
In an interview on RTÉ Radio One's News at One programme, Minister for Food and former Green Party leader Trevor Sargent admitted Ms de Búrca had expressed interest in such a role.
He said Mr Gormley and Taoiseach Brian Cowen were supportive of the proposal, but he insisted it was up to the commissioner, an independent office, to make her own decision.
“It was not something that could be guaranteed or delivered in the way of a party political position,” he said.
The Senator contested the election in Wicklow for the second time in June 2007 but failed to secure a Dáil seat. In August 2007, she was nominated to the Seanad by then-taoiseach Bertie Ahern in the wake of her party's entry into Government.
Ms de Búrca has served as an elected member of the Green Party for eight years on Wicklow County Council. She failed to win a seat as an MEP in the last European elections.
The Green Party leadership has faced severe criticism from its members and supporters since it entered into a coalition Government in 2007.
Just over a year ago three councillors quit amid claims of a growing split between the leadership and grassroots.
When Letterkenny town councillor Neil Clarke resigned in December 2008 he said he had become disillusioned with the party chiefs and felt he could not go before voters again as a Green.
Within weeks, Cork city councillor Chris O’Leary and Dublin representative Bronwen Maher left the party.
Mr O’Leary, who had been one of the most senior party figures in Munster, said he believed the Greens had followed a “stay in government at all costs” agenda.
Ms Maher accused the leadership of unethical conduct and criticised its failure to speak out at Budget cuts.
Additional reporting PA