Keir Starmer QC, the director of public prosecutions, said he had ordered an "urgent examination" of material provided by the police three years ago.
The annoucement came as police said they would not be mounting a new investigation into claims thousands of public figures had their phones hacked.
Mr Starmer said he wanted to reassure himself and the public that "appropriate actions" were taken over the material.
He said he expected to make a further statement on the allegations in "coming days".
He added: "I have no reason to consider that there was anything inappropriate in the prosecutions that were undertaken in this case.
"In the light of the fresh allegations that have been made, some preliminary inquiries have been undertaken and I have now ordered an urgent examination of the material that was supplied to the CPS by the police three years ago.
"I am taking this action to satisfy myself and assure the public that the appropriate actions were taken in relation to that material.
"Given the nature of the offences, the amount of material is, of course, extensive and complex, but it has all been located and a small team is now rapidly working through it.
"This process will need to be thorough, so it will necessarily take some time.
"I am only too aware of the need for urgency and I will issue a further statement as soon as this work has been completed.
"I anticipate being in a position to do so in coming days."
Earlier, Scotland Yard ruled out a fresh probe into the allegations, after new claims were made in The Guardian newspaper today.
Metropolitan Police Assistant Commissioner John Yates said police had seen no additional evidence since its last investigation, which ended with the jailing of News of the World royal reporter Clive Goodman in 2007.
Mr Yates also said detectives had found no evidence that former deputy prime minister John Prescott's phone was tapped.
But he said police would now inform any potential victims that their phone may have been hacked where there was any suspicion.
He said: "No additional evidence has come to light since this case has concluded.
"I therefore consider that no further investigation is required.
"However, I do recognise the very real concerns, expressed today by a number of people, who believe that their privacy may have been intruded upon.
"I therefore need to ensure that we have been diligent, reasonable and sensible, and taken all proper steps to ensure that where we have evidence that people have been the subject of any form of phone tapping, or that there is any suspicion that they might have been, that they have been informed."
Goodman was jailed for four months and private investigator Glen Mulcaire for six months after they were found guilty of phone hacking.
The scandal led to the resignation of then News of the World editor Andy Coulson, who is now the Tories' PR chief.
Conservative leader David Cameron has defended his director of communications as Labour MPs lined up to demand his sacking.
The cross-party Culture, Media and Sport Select Committee announced this morning that it was reopening an inquiry it held after Goodman was jailed.
The committee's chairman, Tory MP John Whittingdale, said it would be asking former News International chief Les Hinton whether he wished to amend his previous assertion that no other journalists knew of Mr Goodman's activities.
He said the committee may also call Mr Coulson to give evidence, among several present and former News International executives.
Those allegedly targeted by the News of the World included former deputy prime minister John Prescott, Mayor of London Boris Johnson and celebrities including actress Gwyneth Paltrow and celebrity chef Nigella Lawson.
Mr Yates said the inquiry had not uncovered any evidence to suggest that Mr Prescott's phone had been tapped.
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