Toto Riina Mafia
Jailed Italian Mafia boss Toto Riina was sent a threatening letterReuters

Italy's most infamous Mafia boss, Salvatore "Toto" Riina, has found himself in the unusual position of receiving a death threat.

The 83-year-old jailed boss was sent a letter warning him to keep quiet after he was taped talking to a fellow inmate about an ongoing trial.

"Shut that damn mouth," read the letter, which was seized by prison authorities. "Remember that your family is free. We'll take care of the rest."

The letter was signed "Armed Falange", a name that was used by diverse terrorist groups in the early 1990s and was later taken over by the mafia, which used it to claim responsibility for its murders.

In recent months Riina, who is serving a life sentence in Milan's Opera prison, was taped discussing with Apulia mafia boss Alberto Lorusso a trial centring on an alleged truce between the government and Cosa Nostra.

The boss of bosses, who is one of the accused at the trial, was recorded as vowing mafia vengeance against the team of prosecutors led by Nino Di Matteo.

"We can't forget this this, Di Matteo," Riina was recorded as saying. "Corleone does not forget."

Di Matteo and his colleagues allege that exponents of the Italian authorities secretly struck a deal with Riina, a native of the rural Sicilian town of Corleone, to end his clan's murderous spree in the early 90s.

Besides threatening prosecutors, in his conversations with Lorusso, Riina, who is known as "the Beast" for his brutality, also bragged about past crimes and criticised the current leadership of Cosa Nostra.

In October he was heard complaining about fugitive mafia boss Matteo Messina Denaro, who, he said, was too concerned about his personal investments in windfarms and had neglected the syndicate.

Riina is standing trial in Palermo together with his right-hand man Bernardo Provenzano and other mafia members, as well as several high-ranking security officials and politicians, including former interior minister Nicola Mancino and Carabinieri general Mario Mori.

They face a wide range of charges including aggression against the state, perjury, and aiding and abetting Cosa Nostra.