Georges Abdallah, a 74-year-old Lebanese teacher who became a left-wing symbol for the Palestinian cause, is to be freed by France on Friday after 41 years in jail.

Described by his lawyer as "the man who has spent the longest time in prison for events linked to the Israeli-Palestinian conflict," Abdallah is expected to be put on a flight directly to Beirut.

Convicted in 1987 for complicity in the murders in France of two diplomats – one American, one Israeli – Abdallah has gradually been forgotten by the wider public.

But his release remained a cause célèbre for activists on the Marxist-Leninist left, with which he still identifies.

His stern-looking bearded face continued to peer from banners in left-wing demonstrations; and once a year protesters gathered to demand his freedom outside his prison in the Pyrenees. Three left-led French municipalities declared him an "honorary citizen".