Iosu Uribetxebarria, a member of the Basque separatist group ETA, was convicted of kidnapping a prison official in Spain more than a decade ago, Radio France Internationale reported Wednesday.
He was transferred from prison to a hospital in northern Spain last week after he went on a hunger strike and refused medical tests related to his cancer.
In Spain, authorities said more than 100 prisoners have gone on a hunger strike in solidarity with Uribetxebarria.
The hunger strike has now spread to 15 prisons in France.
The French prison administration would not say how many prisoners are involved in the strike.