A Turkish court on Thursday decided to free Mehmet Ali Agca "on parole on 12 January", Mustafa Demirbag, his lawyer, told The Associated Press by telephone.
Demirbag said: "We were hiding the decision but it is not a secret anymore."
Earlier, the semi-official Anatolia news agency said a Turkish court has approved the release of Agca, 47, who was extradited to Turkey in 2000 after serving almost 20 years in prison in Italy for shooting and wounding the pope in St Peter's Square in Rome.
John Paul II met Agca in Italy's Rebibbia prison in 1983 and forgave him for the shooting.
Joaquin Navarro-Valls, a Vatican spokesman, said the Vatican would defer to the judgment of the Turkish tribunal.
He said in a brief statement: "The Holy See has learned only from news agencies of the news of the possible freedom of Ali Agca. The Holy See, before a problem of a judicial nature, submits to the decisions of the tribunals involved in this matter."
The Turkish court said he has completed his sentence for unrelated crimes he committed in Turkey.
Agca's family and another lawyer said they were not aware of the court decision. But it was not clear if they were hiding the court decision too.
Dogan Yildirim, a lawyer, said: "I'm surprised. If it's true, justice will finally be served. He has been in prison for so long."
Fatma Agca, Agca's sister, said: "We did not hear it."
The pope met Agca in an Italian
prison in 1983 and forgave him
She was speaking to the AP from the family home in the southeastern city of Malatya.
Agca, a draft-dodger, was expected to be immediately enlisted by the military for obligatory military service when he is released between Monday and Friday, Anatolia said.
Turkish paramilitary police are expected to take Agca first to a local military station and then to a military hospital in Istanbul for medical checks, a routine procedure.
Crimes in Turkey
Upon his return to Turkey, Agca was sent to prison to serve a 10-year sentence for murdering Turkish journalist Abdi Ipekci in 1979. He was separately sentenced to seven years and four months for two robberies in Turkey the same year.
An Istanbul court ruled in 2004 that Agca should only serve the longest sentence - his conviction for killing Ipekci.
In 2004, Yildirim had pressed for Agca's early release in court, calculating that he could be released as early as December 2005 under new laws. But the court had not responded to him then.
Agca served less than six months in a Turkish prison in 1979 for killing Ipekci before he escaped, resurfacing in 1981 in Rome.
"If it's true, justice will finally be served. He has been in prison for so long"
lawyer for Mehmet Ali Agca
Because of the time he had served in the past, the Turkish prison asked for permission from a court to release Agca.
The court ruled that Agca could now be freed within the next week, Anatolia said.
Agca reportedly sympathised with the Grey Wolves, a far right-wing Turkish group that fought street battles against leftists in the 1970s.
He initially confessed to killing Ipekci, one of the country's most prominent left-wing newspaper columnists, but later retracted his statements.