Self-confessed phone hacker Glenn Mulcaire accessed the new identities of individuals put under witness protection but Scotland Yard took no action, it has been claimed.
BBC Panorama reported last night that the Metropolitan Police had evidence in 2006, during a previous investigation into phone hacking, that Mulcaire had accessed the highly secret information.
It claimed that four of those targeted have only recently been told that they were victims, including Robert Thompson, who murdered James Bulger, and fellow child killer Mary Bell.
Ex-Metropolitan police officer Brian Paddick told the programme: "The Witness Protection Scheme is a very expensive operation to give people who've been convicted of very serious offences and people who are very vulnerable witnesses - to give them a completely new identity, so they can have a completely fresh start. For that information to get into the hands of journalists is potentially putting people's lives at risk"
The force insisted that there was no evidence to suggest an officer had leaked the information, and said that instead Mulcaire obtained the information via phone hacking.
Injunctions protect the new identities of certain individuals, including Thompson.
The Attorney General will consider an alleged breach of an injunction by Mulcaire after all criminal trials linked to the current hacking and bribery investigations have finished.
A spokesman said: "In April 2012 the Attorney General's Office was notified by the Metropolitan Police that there was a potential breach of an injunction by Glenn Mulcaire.
"The Attorney General is responsible for taking action when certain injunctions have been breached, but as criminal matters are still ongoing, these cases take precedence."
The former commissioner of the Metropolitan Police, Lord Blair, said that he was not aware that the witness protection scheme had been hacked.
Lord Blair, who led the Met from 2005-08, told BBC Radio 4's Today programme: "I've just heard as of yesterday that the witness protection scheme was targeted by Mulcaire. I do think that was an extraordinary thing to do, and I'm sure that should have been acted upon more than just notifying the people in charge of the witness protection scheme."
Asked if he had known about it, Lord Blair said: "No I did not. It's a very large organisation. It's a very serious allegation, but at the same time the Met at that stage was tackling the most extraordinary problem it had ever come across, in terrorist activities, and that's where my attention was focused.
"I'm not exculpating myself. Somebody should have told me that. I did not hear that.
"But the point about this is that the significance of the investigation now wouldn't have occurred then. We wouldn't have had the resources to do it."
Lord Blair acknowledged that the initial police investigation into News of the World phone hacking should not have been called off following the convictions of Mulcaire and royal correspondent Clive Goodman in 2006.
"I think the criticism of the 2006 investigation is justified," he said. "It was closed down too quickly. I know what the reasons were. The day after the convictions of the two who were convicted of hacking into royal phones was the day that the airliners plot became overt, with people being arrested all over the place.
"It was the same unit investigating it. I think it should have been put to one side and then reinvestigated later on."
Asked whether it was right for the police and prosecutors to devote millions of pounds to investigating and prosecuting breaches of privacy by journalists, Lord Blair said: "Once you get into an investigation of this scale, you are going to follow the evidence, and that's what they have done.
"This was a very serious conspiracy at the heart of the newspaper industry and I do think it was right to pursue it the way they have done."