PRO-LIFE TD Peadar Tóibín has resigned from Sinn Féin.
Mr Tóibín, who had been a member of the party for 21 years, said he took the decision to walk away "with a heavy heart".
He had been suspended by the party for six months after voting against abortion legislation.
During the Republic’s referendum to remove the Eighth Amendment he had campaigned for a ‘No’ vote despite his party backing the 'Yes’ position.
In a statement on Twitter, Mr Tóibín said that in 2012 the Sinn Féin leadership had agreed a written deal that he would not be sidelined due to his views on abortion but that this had been "binned unilaterally" 18 months ago.
"I have lost speaking rights, spokespersons positions, portfolios and have been significantly censored in my engagements with the media," he said.
"These actions have prevented me from fully representing my constituents."
He said he raised issues with the party leadership many times in an effort to resolve them.
"I have strived in every way I know how simply to make space within the party for people with a different view on this one issue," he said.
"For a United Ireland to be achieved, the party must be flexible, broad and inclusive."
The Meath West TD indicated he now intends to build a new 32-county movement.