Barry McElduff has resigned as MP over his controversial Kingsmill video.
Mr McElduff has been under intense scrutiny after posting a controversial video on social media on the anniversary of the Kingsmill massacre. Last week he was suspended from his party on full pay, which sparked further anger and accusations Sinn Fein had not gone far enough.
Michelle O'Neill said Mr McElduff had been a "formidable" public representative for the past 20 years saying he recognised the controversy from his tweet and his continuing role in public office was compounding the distress to the victims of Kingsmill.
Mr McElduff said he did not want to be a "barrier" to reconciliation and healing.
Kingsmill survivor Alan Black welcomed the resignation which came after he gave a powerful interview of his experience of the massacre.
"This past week has been truly awful for me. I am just hanging by a thread. But I am glad he has done the right thing," he said.
DUP leader Arlene Foster said it was right for Mr McElduff to resign saying his party got it "badly wrong".
"Now is the time for Sinn Fein to learn the lessons from these dark events and to deal with the fact that it, and many of its individual members, continue to publicly glorify the murderous deeds of the past," she said.
"This needs to end if we are to build a future based on integrity and respect. Sinn Fein has much work to do to demonstrate they have truly learned from these events.”
Had I been conscious of the connection to the terrible atrocity at Kingsmill I would certainly not have posted that tweet. Barry McElduff
Announcing his resignation as West Tyrone MP, Barry McElduff said: “It is with great sadness that, after more than 30 years as an active Sinn Fein member and public representative I am tendering my resignation as MP for West Tyrone.
“The reason I am doing so is because of the consequences of the Twitter video which has caused such controversy over the last week.
“But the deep and unnecessary hurt this video caused the families of the victims of Kingsmill is my greatest regret.
“I again offer my profound apology to those families and to the wider victims community.
“Had I been conscious of the connection to the terrible atrocity at Kingsmill I would certainly not have posted that tweet. I genuinely did not make that connection, not for a second did I make that connection in my mind."
His resignation comes after he he posted a video posing with a Kingsmill loaf on his head on the anniversary of the 1976 massacre.
Barry recognises that this controversy and his continuing role in public office is compounding the distress to the victims of Kingsmill. Michelle O'Neill
He added: "Kingsmill was wrong, unjustifiable and sectarian. It should never have happened.
“There was no intended reference to Kingsmill in my tweet. But I do accept that there are many people who do not believe this to be the case. I accept also that this view of what happened is deeply damaging to the reconciliation process that is so important to consolidating the peace process and to healing the pain and hurt of the past.
“I cannot undo the pain caused but I know that my continuing role as MP for West Tyrone will compound that sense of hurt and impede any reconciliation process.
“I wish to wholeheartedly thank my family and friends for their steadfast personal support during this difficult time, and the people of West Tyrone whom I have had the privilege to serve as their public representative for over 20 years. I have a deep gra for my native county and its people.
"I am an Irish republican and believe whole heartedly in the reunification of our country and an agreed Ireland in which we heal the wounds of the past together.
“Reconciliation is essential, but that message is not being heard at this time.
“I do not wish to be a barrier to reconciliation and healing and in that spirit I again offer my sincere apologies to the survivors and families of those murdered at Kingsmill.”
Responding, Sinn Fein Northern Ireland leader Michelle O'Neill said: "Yesterday evening, Barry McElduff informed me of his intention to resign as Sinn Fein MP for West Tyrone.
“Barry is doing so as a consequence of the unintended hurt caused to the Kingsmill victims and their loved ones by his recent social media tweet.
“Barry recognises that this controversy and his continuing role in public office is compounding the distress to the victims of Kingsmill, and again offers his profound apology to those families and to the wider victims community.
"He has said that he does not want to be a barrier to reconciliation and I respect that decision.
“Barry has served Sinn Fein and been a formidable champion for the people of West Tyrone at local government, Assembly and Westminster level over the past 20 years and has done so with great commitment, energy and determination.
“For this I want to personally thank Barry and his family, Paula, Niamh, Blannid and Patrick.
“Over the coming weeks Sinn Féin will focus our full efforts on the restoration of the power-sharing institutions on the basis of equality, integrity and respect and fulfil the mandate we received from the electorate in two successive elections last year.”