Veteran UDA figure Sammy Duddy was being treated in intensive care last night after suffering a massive heart attack.
Duddy (62) collapsed yesterday and underwent emergency surgery to remove a blood clot.
He was reported to be in a critical condition in hospital last night.
His wife and children were at his bedside.
A spokesman for the UDA and later their advisors in the Ulster Political Research Group, Duddy has been a prominent loyalist figure for decades.
He currently works for the Conflict Transformation Initiative - the loyalist programme which has had its £1.2m funding threatened by Social Development Minister Margaret Ritchie.
Duddy reportedly moved out of his home in Rathcoole during the summer because of ongoing tensions between the mainstream UDA and the South East Antrim faction.
Those tensions led to violence at the weekend in Carrickfergus. Five men appeared in court yesterday accused of assaults linked to the feuding.
"He has been stressed out lately by everything that's going on," said Frankie Gallagher of the UPRG last night. "We're just hoping and praying he'll be all right.
"This man has fought for peace for years and he's stood up against people who didn't want peace."
Duddy spent 11 months in jail on remand after being arrested during the Stevens Inquiry into collusion between loyalists and members of the security forces. A charge of possessing documents likely to be useful to terrorists was dropped in October 1990.
Duddy unsuccessfully applied for security funding from the Government during an earlier loyalist feud. In 2002, his chihuahua Bambi was killed in a gun attack on his home by loyalists linked to former UDA leader Johnny Adair.
Duddy was also behind a hit loyalist drag act in the Seventies, performing as "Samantha".