Hundreds of serious crimes will go unpunished and many whose sentences have already been confirmed by a lower appeals court will walk free.
Many other cases that are in an earlier phase will also be affected, including the brutal attack by riot police on activists and journalists sleeping in the Diaz school in Genoa during the G8 meeting of 2001, in which dozens were seriously injured. Several of the victims were British.
Next week the trial of the officers blamed for the violence begins in earnest in Genoa - but the new law is likely to render it null and void. The trial will get under way, but will never finish. No one will be punished.
The Bill has been nicknamed the "Save Previti Law" by opponents of Prime Minister Silvio Berlusconi, who claim it is just the latest case in which the media magnate has misused his position to allow him and his friends to dodge justice.
Cesare Previti, 70, who served as minister of defence in Mr Berlusconi's first, short-lived government in 1994 and is now a senator, was for many years one of his lawyers. Two years ago he was convicted of bribing judges in Rome to ensure that a takeover battle in which Mr Berlusconi was engaged before he entered politics was not stymied by the courts. He was found guilty and given an 11-year sentence, reduced to seven years on appeal.
Loyalty to his friends is one of Mr Berlusconi's most remarkable qualities. Many of his closest associates have been with him for decades. He has stood by figures such as Marcello dell'Utri, the founder of his party Forza Italia, who underwent a long trial in Sicily for Mafia association, and made no effort to distance himself when dell'Utri got a nine-year sentence last year.
Mr Berlusconi has gone to huge lengths to save Mr Previti from prison. He has induced his Justice Minister Roberto Castelli to force the "Save Previti Law" through parliament while other matters, including the chaos at the Bank of Italy, remain to be dealt with.
But this week the Court of Cassation spelt out what the new law, which cuts in half the time within which convictions for many different offences will be "prescribed" or struck down, will mean in practice.
The court believes that of 3,365 cases pending, as many as 1,652 may be killed if the Bill becomes law. They include cases of manslaughter, corruption, embezzlement and family abuse. Nearly 90 per cent of corruption cases before the court will be struck down. "We will be able to finish our work before lunch," was one judge's sardonic comment.
Rome's La Repubblica newspaper has been examining the consequences of the new law and reports that two of the trials involving excesses committed by riot police in Genoa are among those likely to be killed off by it.
Other notorious cases for which justice will now never be done include the collapse of a block of flats in Rome in 1998 in which 27 people died, four of them children. Two men have been convicted and sentenced, but thanks to the new law the convictions will die before they can be confirmed by Court of Cassation. A major case of corruption in the awarding of contracts for building Italy's high-speed railway, in which 30 people are charged, and the defrauding of 100 families in Rome, many of whom lost all their savings, in a housing scam, may also be affected.
Mr Castelli claimed that the data cited by opponents of the Bill was "untrustworthy", and insisted it was "a good law". But magistrates condemned it as "a permanent crypto-amnesty".