The bill, which was approved by the cabinet last week and is waiting for a green light from the prime minister's office, is a bid by the government to stop potentially incriminating but private conversations being splashed in the media.
"It will be the last rites for criminal trials," warned Giulia Bongiorno, one of Italy's most renowned lawyers.
Under the proposed law, police officers listening to wiretapped conversations will only be allowed to transcribe and pass on to prosecutors "relevant" bits. Any transcripts not used in trial would be sealed as "secret".
Successive governments have attempted to change the law to protect against what former prime minister Silvio Berlusconi in 2010 dubbed "porno politics", where reputations are ruined and trials jeopardized before courts rule.
But each time their efforts have been stymied by police, prosecutors and the media, who say it would hobble investigations and limit journalists' ability to report on matters of public interest.
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Justice Minister Andrea Orlando said on Thursday the bill would "in no way hamper the possibilities for prosecutors and police to use wiretaps as instruments in their investigations".
"Nor does it affect freedom of the press or the right to inform," he said.
But La Repubblica daily said Sunday it would mean "six months to three years for the journalist who, doing his job, finds and publishes wiretaps that the prosecutor considers 'irrelevant' for trial, but are extremely relevant for their news value".
"Publishing such transcripts will become extremely risky, with the real chance of ending up behind bars for revealing 'secret' wiretaps that the political sphere above all does not want to end up in the papers," it added.
Leaving it up to policemen to decide what could be of interest to the legal process was pure folly, Bongiorno said.
"There will be an enormous amount of discretion in deciding, almost arbitrarily, what wiretaps are relevant. Overworked Italian judges won't be able to perform the necessary controls this law demands," she said.
Bongiorno argued that Orlando had also ignored the "most obvious pitfall: the ambiguity of language".
"Perhaps he thinks that people always talk openly? Does he know how many times the word mozzarella is used instead of cocaine? Will mozzarellas be held to be relevant or not?" she said.
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In one famous trial in 1983, judges convicted dozens of defendants for mafia crimes after examining transcripts of conversations in which "horses" was used as a code word for heroin and "green lemons" for US dollars.
Former anti-mafia magistrate Antonio Di Pietro said the bill "has big holes in it" and called for it to be modified. "The criticisms raised by magistrates, lawyers, jurists and journalists should convince the government to review the bill and persuade parliament to call for radical changes," the Italian National Press Federation (FNSI) said.
Transforming it into law not only risked "serious conflicts with heavy consequences for freedom of the press" but would also deny citizens "the right to know about mafia, corruption and crime cases," it said.
By Ella Ide