Cospito, 55, has been on a hunger strike for more than four months against the tough 41 bis prison regime he is being held under, normally reserved for mafia bosses.
He has lost more than 50kg and there are fears about his worsening physical condition.
He was moved back from Opera Prison to Milan's San Paolo Hospital after he stopped taking vital supplements after the supreme Cassation Court on February 24 rejected his plea against the 41 bis regime.
It is the second time he has been returned to hospital from the Milan jail.
Anarchists have been staging a wave of arson and vandalism attacks inside and outside Italy in support of Cospito, who is serving a total of 30 years in jail, 20 for attacking a Carabinieri barracks and 10 more for kneecapping a nuclear plant manager.
Cospito was sentenced to four years of 41 bis last May.
His supporters have accused the State of torturing him.
International rights groups have described the 41 bis regime as inhumane.
Italian officials including Prime Minister Giorgia Meloni have said the State will not negotiate with terrorists and have stressed the importance of 41 bis in weakening Cosa Nostra.
In rejecting his plea, the Cassation Court also said that he could run operations from jail if he were returned to normal custody.
He is campaigning for the 41 bis to be abolished for all inmates, including mafia dons.
ANSA
