A doctor monitoring the health of a jailed anarchist hunger striker in Greece says the 29-year-old man is at the "final stage" of life and appealed to the government to secure his release.

Kostas Sakkas was arrested in 2010 in terrorism-related charges but has been held in prison in pre-trial detention for longer than the 18-month limit specified by the Greek constitution.

Sakkas denies the charges, and supporters of a campaign to release him, including prominent politicians and academics, said Wednesday that they would start their own hunger strike next week unless he is released.

Sakkas began his hunger strike on June 4 and is currently in a state hospital in Athens under police guard.