The Sinn Féin Dublin City Councillor Killian Forde has resigned from the party.
The Party's Dublin Chairperson described the move as a "disappointment" and called on the councillor to resign his seat to allow a Sinn Féin co-option in the Donaghmede ward.
Killian Forde recently defied the party by backing Dublin City Council's budget.
He is the third city councillor to resign from Sinn Féin.
Dublin Sinn Féin Chairperson Eoin Ó Broin said it had become clear in recent times that Killian Forde was "in the wrong party".
"This was most graphically illustrated at the December meeting of Dublin City Council when Killian Forde, in defiance of a democratically agreed Sinn Féin position, voted in favour of the Council Estimates for 2010 which imposed new refuse charges on 40,000 low income households," he said.