Peadar Tóibín has resigned as a member of Sinn Féin, saying he took the decision "with a heavy heart".
The Meath West TD had been suspended from the party for six months after voting against abortion legislation.
Mr Tóibín said that in November 2012 Sinn Féin leadership had agreed a written deal with him that stated he would not be marginalised due to his views on abortion.
He said this deal worked well for four years, but was "binned unilaterally" 18 months ago.
"I have lost speaking rights, spokesperson's positions, portfolios and have been significantly censored in my engagements with the media.
"These actions have prevented me from fully representing my constituents," he said.
Sinn Féin leader Mary Lou McDonald said she learned of Mr Tóibín's resignation "with regret".
However, she said all of the party's elected representatives are bound by the decision of the Ard Fheis.
"The people voted to repeal the Eighth Amendment. There is a responsibility on all Sinn Féin TDs to give effect to the people's vote and to represent Sinn Féin policy," she said.
"Unfortunately Peadar was unable to do this and was suspended from the party and has now taken the decision to resign.
"I wish Peadar and his family well for the future."
It is with a heavy heart that I resign from Sinn Féin today. I have been a member of the party for 21 years. In that time I poured all my efforts into achieving Irish Unity & Economic Justice. This clearly is no longer enough. I will now help to build a new 32 County movement. pic.twitter.com/RqzimdR8Zp— Peadar Tóibín (@Toibin1) November 15, 2018
Mr Tóibín, a member of Sinn Féin for 21 years, had actively campaigned for a No vote during the referendum on the Eighth Amendment, even though his party had supported a Yes vote.
The Meath West TD has been suspended from the party twice.
He was suspended for six months in 2013 after voting against the Protection of Life During Pregnancy Bill.
In his statement, which was issued on Twitter, he said he raised issues with party leadership "many times" in an effort to resolve them, with no success.
He said: "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.
"For a United Ireland to be achieved, the party must be flexible, broad and inclusive."
Mr Tóibín indicated he now intends to build a new 32-county movement.
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