The MP for West Tyrone, Barry McElduff, has resigned as a result of a controversy over a Twitter video.
The Sinn Féin MP posted a video of himself in a shop with a loaf of Kingsmill bread on his head.
It was published on the 42nd anniversary of the Kingsmills murders, in which 10 Protestant workmen were shot dead by the IRA.
The sole survivor of the attack said he would not accept his apology adding: "I'll never forgive him".
Mr McElduff has said that he did not make a connection in his mind between the video and the massacre, but he accepted many people do not believe that.
Alan Black, who was the only person to survive the 1976 attack, welcomed the resignation.
"I'm not on a crusade. I've done what I've done up to now to protect the memory of the boys and to try to protect their families," he told the BBC.
"But he did the right thing stepping down - eventually. I won't be speaking publicly about Kingsmills again unless there is some development at the inquest.
"As far as I'm concern the Barry McElduff incident is over now. But I don't accept his apology. I'll never forgive him."
Mr McElduff said the "deep and unnecessary hurt this video caused the families of the victims of Kingsmills is my greatest regret" and reiterated his apology to them.
"Kingsmills was wrong, unjustifiable and sectarian. It should never have happened," he said.
He said that the view that the video was intended to refer to the massacre would "impede any reconciliation process".
Analysis: Mark Devenport, BBC News NI political editor
Barry McElduff has consistently protested that he never intended to cause offence with his Kingsmills loaf video, and that its publication on the 42nd anniversary of the Kingsmills atrocity was just a terrible coincidence.
With that in mind, his family and friends rallied to his defence and he must have initially hoped that his three month suspension would buy enough time for emotions to cool.
However, the criticism never abated, with Sinn Féin's opponents accusing the party of not matching their initial tough words with appropriate action.
Read more here.
"I am an Irish republican and believe wholeheartedly 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," Mr McElduff added.
The former UUP MLA Danny Kennedy, who is a cousin of Mr Black, said Mr McElduff had "bowed to the inevitable".
He told BBC Talkback it was necessary for Mr McElduff to step down, and said today was "not a day of celebration".
"There is nothing here to celebrate except that at long last a public representative has owned up and taken responsibility for his own actions," he said.
The DUP leader, Arlene Foster, said it was right that Mr McElduff resigned saying he was not fit for public office and should have resigned in the immediate aftermath of posting the video.
She accused Sinn Féin of failing to deal with the situation.
"By merely suspending him and continuing to pay him, they compounded his disgraceful actions and demonstrated a lack of respect and compassion for the victims of Kingmills and indeed victims more widely.
"Now is the time for Sinn Féin 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.
Ulster Unionist leader Robin Swann welcomed Mr McElduff's resignation but said that "it should have happened last week".
He called for "a non-partisan candidate who will be a voice for victims to contest this seat against Sinn Féin".
"If a candidate emerges that allows cross-community support to coalesce around, it would send a strong message that this type of behaviour will not be tolerated," he said.
The DUP's Gregory Campbell was cautious about the suggestion, commenting that Sinn Féin had achieved almost 51% of the vote in the recent general election.
"Whatever decisions we may make separately, let's not do anything to turn Sinn Féin into a victim," the East Londonderry MP said.
"They were the perpetrators of this incident, not the victims of it," he added.
Sinn Féin's northern leader Michelle O'Neill said Mr McElduff told her of his intention to resign on Sunday evening.
"Barry recognises that this controversy and his continuing role in public office is compounding the distress that's been caused to families and the victims of Kingsmills and he again offers his profound apology for the unintended hurt that has been caused to both the Kingsmills families, but also to the wide victims community," she said.
"He has said that he does not want to be a barrier to further reconciliation and I respect that decision," Mrs O'Neill added.
She also called for Sinn Féin's Máirtín Ó Muilleoir to "reflect on what he does on social media" after he retweeted Mr McElduff's video.
Mr Ó Muilleoir has apologised for retweeting the video, saying he saw it as being "wholly apolitical and retweeted it on that basis".
Mr McElduff's resignation will trigger a by-election in West Tyrone, where last year Mr McElduff had a majority with more than 50% of the overall vote.