Carlos the Jackal is on a hunger strike to protest against being held in isolation.
Cold War-era criminal mastermind Carlos, whose real name is Ilich Ramirez Sanchez, is serving a life sentence in a French prison for the murder of three people in Paris in 1975.
He goes on trial next month, accused of killing 11 people in four attacks in France in the 1980s.
His lawyer, Francis Vuillemin, wrote in a letter to the French justice minister that Carlos started a hunger strike on October 18.
The prison authority said Carlos's rights were being respected.
Carlos gained notoriety during the Cold War as the mastermind of deadly bombings and hostage dramas, including an assault on Vienna's Opec headquarters.