Green Party Leader John Gormley has dismissed Déirdre de Búrca's criticisms as 'entirely without foundation'.
Ms de Búrca used her resignation from the Green Party and Seanad Éireann this morning to attack the party.
Mr Gormley said: 'You only had to look at the policies being implemented over the last week to see how much influence the party is exerting on Government.'
He said they were united as a parliamentary party and had received an overwhelming endorsement from the members for the new Programme for Government.
Mr Gormley confirmed that Ms de Búrca had expressed an interest in a job in EU Commissioner Máire Geoghegan-Quinn's cabinet but this had not been possible.
Asked if he thought it explained the timing of the resignation, Mr Gormley said that was a question for Ms de Búrca.
Speaking later on RTÉ's Six-One News, Mr Gormley said that he was not afraid to face the electorate and reiterated his view that the Green Party is doing an effective job in Government.
In a letter of resignation to Green Party leader Mr Gormley, Ms de Búrca said: 'I can no longer support the Green Party in Government, as I believe that we have gradually abandoned our political values and our integrity and in many respects have become no more than an extension of the Fianna Fáil party'.
Read Ms de Búrca's letter
Ms de Búrca claimed Fianna Fáil is 'running rings' around the Green Party and continues to take advantage of its inexperience.
In the letter, she says the Green Party has been 'paralysed' by the rejection of several candidates in the local and European elections last year.
Ms de Búrca stood in the European parliament elections in the Dublin constituency but failed in her attempt to get elected.
In the letter, Ms de Búrca says she has lost confidence in Mr Gormley. She adds that in his role as leader, he has done a disservice to the party and its members by allowing a 'drift' to occur.
Gogarty hits out at comments
Green Party TD Paul Gogarty accused his former colleague of making 'outrageous' and incorrect claims about the party.
Mr Gogarty said the claims were 'clearly designed to deflect from the real reasons' for her resignation. He said there was no question over the leadership of John Gormley following the resignation.
The TD said the party were 'shell-shocked' by her announcement, but he said the Green Party was a united party.
The Green Party issued a statement on Ms de Búrca's resignation, saying that they are 'disappointed' by the decision.
Read the full statement from the Green Party
The statement said that the party was aware that she had been unhappy with her situation for some time.
The statement went on to reiterate that the Greens intend to stay in Government saying: 'We have come to do a job and as long as we are getting that job done we will stay. If we are not making progress, we will think again.'
On RTÉ Radio's News at One, Green Party Junior Minister Trevor Sargent said he was very disappointed by Ms de Búrca's resignation, but he said her criticisms of the party leader 'didn't ring true' and were 'very unfair'.
Minister for Communications Eamon Ryan said that Mr Gormley retains the support of the parliamentary party.
Earlier, Green Party TD Ciarán Cuffe wished Ms de Búrca well via his Twitter page saying that she 'has been a great campaigner'.
Cowen to select replacement
There will be no by-election to the fill the Seanad seat left vacant by Ms de Búrca's resignation.
Because Ms de Búrca was nominated by Taoiseach Brian Cowen, he will also appoint her replacement.
The Cathaoirleach of the Seanad, Senator Pat Moylan, is to write to Mr Cowen next Tuesday to formally notify him of the resignation.
There is no deadline by which a replacement must be made.
