The Italian judge who investigated the 1981 murder attempt on John Paul II has warned the Turkish would-be assassin that his life will be "in grave danger" when he is released from jail because he "knows too much".

Mehmet Ali Agca, 48, is to be released from Kartal high security jail in Turkey for good behaviour, perhaps as early as tomorrow.

He served 19 years of a life sentence in Italy for the assassination attempt before being pardoned by the late Pope in 2000. He was then extradited to Turkey to serve a separate ten-year sentence for murdering a Turkish journalist in 1979.

A clearly irritated Cardinal Angelo Sodano, the Vatican Secretary of State, said of the news: "No one told us anything." But Monsignor Stanislaw Dziwisz, John Paul’s former secretary and now archbishop of Krakow, said the late Pope would have approved of the early release. "He is praying for him from heaven, and I am too," Monsignor Dziwisz said.

Ferdinando Imposimato, the retired judge who led the initial inquiry and has since conducted his own research, said that 25 years after the shooting in St Peter’s Square on May 13, 1981, "many mysteries remain".

He told The Times that he remained "120 per cent convinced" that the murder had been "planned in Moscow".

"The Kremlin started to plot the Pope’s murder the moment he was elected in October 1978," Signor Imposimato said.

Soviet leaders had sought - in vain - to prevent the Polish Pope from inspiring the anti-Communist revolt in his native country which in the end sparked the collapse of the Soviet bloc in 1989.

The untold secrets however included the alleged involvement in the conspiracy of East European agents in the Vatican as well as of their Bulgarian and Turkish accomplices, and the alleged presence on the square of other gunmen. "I am convinced that once he is free, Agca’s life will be in grave danger because he knows many truths about the plot," the judge said.

Agca, a "cold-eyed killer" and a member of the extreme right wing Turkish Grey Wolves, was arrested on St Peter’s Square minutes after the shooting with the smoking gun in his hand.

A court sentenced him to life imprisonment two months later. Three Turks and three Bulgarians went on trial in 1986 for involvement in the plot, but were acquitted for lack of sufficient evidence.

In his memoir Memory and Identity, published shortly before he died last year, John Paul II - who forgave Agca and prayed with him in his cell in 1983 - said that his life was saved by "divine grace".