Willie Frazer has lost his fight for life.
His wife Anne revealed last week that he has been in and out of Craigavon Area Hospital recently after suffering organ failure.
Belfast Live reports he passed away at 3.55pm.
The 58-year-old from Markethill, Co Armagh , led victims' group the Family Research and Policy Unit (FRPU).
Last Friday it was confirmed that he was in intensive care.
His wife Anne said at the time: "Willie is critically ill and it is all in God’s hands. We would like to thank all the well wishers for all their messages of support and prayers."
Newry & Armagh DUP MLA William Irwin has shared his condolences with the Frazer family.
He said: "I visited William only a few days ago and chatted with him as he lay on a hospital bed and it is very tragic that he has sadly passed away after this period of ill health.
"I had known William for many years and everyone will be aware he had suffered a lot through the loss of his father and other family members directly as a result of IRA terrorism.
"He will be sadly missed by his wife and family circle and I extend my deepest sympathy and prayers to them as they come to terms with this awful loss.
"William was a tireless advocate for innocent victims of republican terrorism and his contribution in assisting many victims through various measures is something that they will be forever grateful for," he added.
Jamie Bryson took to Twitter to share his heartbreak. He said: "Heartbroken. Farewell my friend."
DUP Cllr Dale Pankhurst tweeted: "Deepest sympathy and condolences to the family of William Frazer who passed away this afternoon."
