Dublin City councillor Bronwen Maher has become the second senior figure to resign from the Green Party this week.
She has claimed that in government it has become a "centrist lifestyle-lite Green party" with which she no longer felt a connection.
Ms Maher, who has been a member of the Greens for 20 years and a councillor since 2004, wrote an open letter to party leader John Gormley this morning tendering her resignation.
She is the second councillor to leave the party this week, following the resignation of Cllr Chris O'Leary in Cork.
At a press conference held later this morning Ms Maher said she could no longer support a party that had a "weak policy platform" in Government and whose role in the coalition with Fianna Fáil and the Progressive Democrats had turned it into a party that was "irrelevant and out of touch".
Ms Maher, whose political base is in Clontarf, read out the open letter which was sharply critical of the party's record in Government over the past 18 months.
She said that in opposition the party had strong policies on the environment, human rights, social justice issues and the local community.
She listed campaigns such as the Poolbeg incinerator, the M3 and Tara, the Shannon stop-over, and the support of the Greens for the Rossport Five protest over the locations chosen for the Corrib gas project.
She claimed that the Greens had given up a strong opposition role to implement what was, largely, EU policy that would have been implemented in any instance. She said that planning, environmental, green energy and waste policy initiatives had all come from the EU.
In particular, said Cllr Maher, the Greens had failed to persuade Fianna Fáil to introduce a carbon levy or tax at an early stage. It had also failed to prevent the cut-backs in the Equality Authority or prevent the withdrawal of vaccines for women.
Ms Maher also criticised the narrowness of the Green agenda in Government saying that most of its achievements were confined to the policy areas of its two ministers, Mr Gormley and Eamon Ryan and that other Green priorities like equality, social justice and education had been rejected.
"We are corralled in these two policy areas. We are told that Ministers are working silently behind the scenes [in other policy areas]. That is an excuse."
Ms Maher was in the minority of 13 per cent who voted against the party going into Government on the grounds that the party had failed to secure an ambitious policy agenda for coalition.
In response to suggestions that her departure was inevitable – and welcome by some within the Greens – she said: "It's an indication of where we are. Let us get rid of the squeaky wheels."
She denied that her departure was motivated by concerns about the upcoming elections and an attempt to save her council seat. She said her chances were the same inside or outside the party.
Ms Maher added that she would stand as an independent and there were no plans to form any alliance with other groups or disaffected Green party members.