Progressive Unionist Party leader David Ervine has died in the Royal Victoria Hospital in Belfast.
David Ervine became Progressive Unionist leader in 2002
Mr Ervine, 53, was taken to hospital after suffering a heart attack at his home on Sunday. He later had a stroke and a brain haemorrhage.
Mr Ervine, a former UVF prisoner, was a key figure in brokering the loyalist paramilitary ceasefire of 1994.
In a statement on Monday afternoon, his family said he had "passed away quietly with peace and dignity".
"The family would like to thank the Royal Victoria staff for all that they've done," they added.
Mr Ervine was originally taken to the Ulster Hospital in Dundonald but was then transferred to the intensive care unit at the RVH.
The PUP leader had been an assembly member for East Belfast since 1998 and also represented the Pottinger area in Belfast City Council.
Prime Minister Tony Blair said he was very sorry to hear of Mr Ervine's death.
"David was a man who, whatever his past, played a major part in this last 10 years in trying to bring peace to Ulster," Mr Blair said.
Taoiseach Bertie Ahern said the PUP leader had been a courageous politician.
"David Ervine had travelled his own difficult journey to democratic politics but he made that journey," Mr Ahern said.
Northern Ireland Secretary Peter Hain said Mr Ervine was a "talented, courageous and an honest politician".
"He wanted to solve problems not simply restate them, always looking for a way forward," Mr Hain said.
Ulster Unionist leader Sir Reg Empey said Northern Ireland had lost "a unique, charismatic and uncharacteristically spin-free politician".
"He realised that violence belonged in the past and was keen to play his part in helping loyalists make the transition towards exclusively peaceful and democratic means," Sir Reg said.
Sinn Fein leader Gerry Adams he was shocked by Mr Ervine's death.
"He made a valuable and important contribution to moving our society away from conflict," Mr Adams said.
DUP deputy leader Peter Robinson, the MP for East Belfast, said Mr Ervine was a "staunch defender" of the area.
"Even those who saw politics from a different angle of vision would openly acknowledge that he genuinely wanted to see a new era of peace and stability in Northern Ireland," he said.
SDLP leader Mark Durkan said the PUP leader "was a great character and he always showed real character".
"While his strong voice will no longer be heard his strong views and values will continue to shape the future for which he worked so hard," Mr Durkan said.
The PUP said loyalism had lost its most articulate spokesperson.
"Unionism has lost the most progressive voice of this generation. Politics has lost a statesman. Our peace process has lost its most optimistic advocate and Ulster has lost a devoted son," a party statement said.
Alliance Party leader David Ford also paid tribute to Mr Ervine.
"His personal passion for progress will be missed. He played a pivotal role in turning loyalism away from violence," Mr Ford said.
Peter Bunting, of the Irish Congress of Trade Unions, said Mr Ervine was "one of the finest and bravest of his generation".
"All working class people in Ireland and Britain have lost a true friend," Mr Bunting said.
Mr Ervine, who was married with two sons, became the leader of the PUP in 2002 after replacing Belfast councillor Hugh Smyth.