A former IRA terrorist said Jeremy Corbyn's support for the group gave its members "great encouragement" and claimed the Labour leader and shadow chancellor John McDonnell have "blood on their own hands".
Sean O'Callaghan said the support of the Labour leader and Mr McDonnell had "made it easier" for the republican terror group to carry out atrocities.
A spokesman for Labour dismissed the "false and contemptible claims" and stressed Mr Corbyn and Mr McDonnell's commitment to the peace process.
IRA terrorist-turned-informer O'Callaghan, a convicted double killer, said he witnessed the impact of the support from the Labour figures during the 1980s.
Writing in The Sun he said: "IRA men and women, many young and hopelessly politically naive, derived great encouragement from the solidarity openly displayed by Corbyn, McDonnell and their associates.
"I know. I was there. I witnessed the effect.
"They might not have pulled a trigger or planted a bomb but they certainly made it easier for those who did.
"By boosting our morale, they prolonged the violence and without a doubt for that, have blood on their own hands."
Mr Corbyn's spokesman said: "These are false and contemptible claims by a convicted murderer. John and Jeremy consistently campaigned for peace and championed the Good Friday Agreement."
The Labour leader has faced repeated questions about his association with the IRA and its political wing Sinn Fein during the 1980s and 1990s.
On Monday he condemned "all acts of violence from wherever they came" during the Troubles Ireland, but declined to specifically denounce the IRA as terrorists.
The Labour leader spoke after Northern Ireland Secretary James Brokenshire accused him of having "IRA sympathies" and urged him to make clear whether he believes the group were terrorists and if their actions should be "unequivocally" condemned.
Earlier, Mr Corbyn's shadow defence minister openly contradicted him. Nia Griffith said she "unequivocally condemns" IRA bombings in Northern Ireland.
Although she also added she was "not going to speak for" Mr Corbyn, Ms Griffith said: "I had friends who actually served in Northern Ireland - I absolutely and unequivocally condemn IRA bombings in Northern Ireland.
"In terms of our service personnel who served there, no way should they be subject to vexatious claims, long drawn out processes referring back to times which actually in many instances have already been dealt with and have already been examined fully."
Mr Corbyn yesterday side-stepped a series of direct questions from Secretary of State James Brokenshire after he put Northern Ireland front and centre in the General Election battle by bluntly asking if Mr Corbyn believes the IRA's campaign was legitimate.
"Their complete failure unequivocally to condemn terrorism, and to attempt to contextualise it, are deeply worrying coming from two people who in just over two weeks seek to be entrusted with the security of the United Kingdom," said Mr Brokenshire.
And he added: "If they are unable unequivocally to condemn IRA terrorism, do they actually believe that the IRA campaign, or as they would put it 'armed struggle', was both justified and legitimate?
"These questions can easily be answered with a straight yes or no. Failure to do so will lead people to draw their own conclusions." Mrs Foster, meanwhile, said Mr Corbyn had been close to the political representatives of the IRA at the height of the Troubles.
"While Theresa May is well within the political mainstream and has proven herself to be a solid and reliable unionist, Jeremy Corbyn is beyond the proverbial and the political pale," she said in a speech in London.
"It is hard to take seriously the proclaimed unionism of a man who was so close to the political representatives of the IRA at the height of the Troubles.
"It is hard to see much good coming for the Labour Party from the coming election except the replacement of their party leader."
Mr Adams, however, suggested Mrs Foster was "lending herself to a complete distraction" and said: "Jeremy Corbyn was on the right side of history."
Mr Corbyn said: "I condemn all acts of violence in Northern Ireland from wherever they came.
"I spent the whole of the 1980s representing a constituency with a large number of Irish people in it, we wanted peace, we wanted justice, we wanted a solution.
"The ceasefire, the first ceasefire, helped to bring that about, and bring about those talks, which were representative of all sections of opinion in Northern Ireland."