David Ervine, one of Northern Ireland leading loyalist politicians, was tonight fighting for his life in hospital.
Sources said Mr Ervine had suffered either a heart attack or a stroke overnight. His family was at his bedside in a Belfast hospital.
A spokesperson for the Progressive Unionist Party, which he leads, said their thoughts and prayers were with the family. Mr Ervine is the Assembly member for East Belfast.
Mr Ervine, 53, was originally treated in the Ulster Hospital but was transferred later today to the Royal Victoria.
The PUP's Northern Ireland Policing Board member Dawn Purvis said: "David took ill this morning and is being cared for in hospital. His family are with him.
"Obviously our thoughts and prayers are with David and his family at this difficult time."
Assembly colleagues in other parties and on Belfast City Council were tonight monitoring the situation and wished Mr Ervine well.
Messages of support and concern were also sent to the PUP leader's family by Northern Ireland Secretary Peter Hain and Irish Foreign Minister Dermot Ahern.
The British Home Secretary and former Northern Ireland Secretary John Reid said his thoughts and prayers were with David Ervine's family at this difficult time.
PA