This handout video grab released by the Italian Carabinieri Press Office shows the transfer of Matteo Messina Denaro (C) to an undisclosed location. AFP

The 61-year-old has been suffering from colon cancer for several years. It was his decision to seek treatment that led to his arrest following a visit to a clinic in the Sicilian capital Palermo.

Facing numerous life sentences, he was detained in a high-security jail in L'Aquila, where he continued treatment in his cell, according to reports.

But in early August, Messina Denaro was moved to the inmates ward of the local hospital, where his condition has declined in recent days.

He is now in an "irreversible coma," according to media reports. Medics have stopped feeding him and he has asked not to be resuscitated.

Messina Denaro was for many years a leading figure in Cosa Nostra, the real-life Sicilian crime syndicate depicted in the Godfather movies.

He was also one of its most ruthless bosses and was handed six life sentences over the years, including for his role in the murder of anti-Mafia judge Giovanni Falcone in 1992.

He was also found guilty of involvement in a string of deadly bombings in Rome, Florence and Milan in 1993, and the kidnapping and subsequent murder of the 12-year-old son of a witness in the Falcone case.

Messina Denaro disappeared in the summer of 1993, and became the top name on Italy's most-wanted list.

There was intense speculation in the years that followed about where he had gone. In the end, he was found to have been staying near his hometown of Castelvetrano in western Sicily.

Investigators had been combing the Sicilian countryside for possible hideouts for years, wiretapping members of his family and his friends.

They were heard discussing the medical problems of an unnamed person who suffered from cancer, as well as eye problems -- a person who detectives became sure was Messina Denaro.

They used a national health system database to search for male patients of the right age and medical history, and eventually closed in.

But while his arrest brought some relief for his victims, the mob boss always maintained his silence.

In interviews in custody since being arrested, Messina Denaro even denied he was a member of the Cosa Nostra.

Agence France-Presse