Paddy Hill, one of six men wrongly convicted of the 1974 Birmingham pub bombings, has died aged 80.
In a post on Facebook, the Miscarriages of Justice Organisation (MOJO) said Mr Hill died peacefully at home on Monday morning.
Explosions at two Birmingham city centre pubs - the Mulberry Bush and Tavern in the Town - killed 21 people in November 1974.
The Birmingham Six spent 17 years in prison for the bombings before their convictions were quashed in 1991.
Mr Hill founded the Glasgow-based MOJO after his release, helping others who were imprisoned despite being innocent of the crimes they were accused of.
MOJO said its condolences were with Mr Hill's family "at this sad time".
"We ask that you respect the family's privacy," the statement added.
"May he rest in eternal peace."
Speaking in 2017, Mr Hill said an apology from a man who said he was part of the group who planted the bombs was insulting to those bereaved.
The other members of the Birmingham Six are Gerry Hunter, Johnny Walker, Hugh Callaghan, Richard McIlkenny and Billy Power.
Mr Callaghan died in 2023 aged 93, and Mr McIlkenny died in 2006 aged 73.