Seamus Mallon, the former Stormont deputy first minister and SDLP deputy leader, has died at the age of 83, his party has announced.
Mr Mallon was one of the architects of the 1998 Good Friday agreement that brought peace to Northern Ireland after years of sectarian conflict.
The resident of Markethill, Co Armagh, has been hailed as a champion of justice and reconciliation - with Tony Blair calling him "one of the most important architects of peace in Northern Ireland".
The SDLP's leader Colum Eastwood said his predecessor was "was an Irish Patriot" and "a force of nature".
He added: "In the darkest days of conflict, when hope was in short supply, Seamus represented the fierce thirst for justice that ran through the SDLP and through communities that had lost so much to political violence.
“His passion for peace underpinned by truth, justice and reconciliation came from a lifetime as a proud son of Markethill where he was born, grew up and raised his own family.
"It didn’t matter who you were, where you worshipped or what your politics were, there was always help to be found at Seamus’ hearth.“
Mr Mallon was deputy first minister of the first devolved Assembly at Stormont following the 1998 Good Friday Agreement, leading the powersharing coalition government along with first minister Lord Trimble of the Ulster Unionists.
"I spent many hours listening to him and learning from him" Tony Blair said in a statement. "He had a brilliant turn of phrase and sharp wit which he would use to great effect. He could be difficult but never ill intentioned".
Meanwhile former US President Bill Clinton called Mr Mallon a "hero of the peace process in Northern Ireland".
"From his earliest entry into politics, Seamus never wavered from his vision for a shared future where neighbours of all faiths could live in dignity-or from the belief he shared with John Hume and the entire SDLP that nonviolence was the only way to reach that goal," he added.
"As his party's chief negotiator in the talks leading to the Good Friday Agreement, he was respected by all parties for his intelligence and integrity, his candour and convictions."
Alastair Campbell, Mr Blair's former spin doctor, went on to praise Mr Mallon as "One of the nice guys - and one of the great guys when it came to the peace process"
He added: "He did not always get the biggest headlines because he didn't shout the loudest. He had commitment and intelligence to match.
"He was motivated by the right things and went about them in the right way. He was a man of passionate views and beliefs always rooted in the desire to improve the lives of the place and the people he came from. But he was pragmatic not ideological in his pursuit of them.
"My last dealings with him were over a little film we made for the peoplesvote-uk campaign. We lost that one but it showed he fought to the end for what he believed. And his life and career as a whole were a huge success. His positive legacy in Ireland is immense. RIP."