Police will leave "no stone unturned" in their investigation into allegations of phone hacking at the News of the World (NoW), Scotland Yard's head says.
Appearing before the Metropolitan Police Authority, Acting Commissioner Tim Godwin defended the force's handling of the case so far.
On Wednesday the Met said it had received "significant new information".
There has been criticism of Scotland Yard's handling of the case from figures including Lord Prescott.
The Met has been accused of failing to inform many of the alleged victims of phone hacking when they recovered files that referred to a long list of public figures.
Speaking about the investigation, Acting Commissioner Godwin told the panel: "It will be very robust and it will be under scrutiny as it should be.
"It will restore confidence in victims who feel they have not been given a service. It will be with no stone unturned. We have some of the most skilled investigators in the country and you will be proud of what they do."
He added that the force was not afraid to be held accountable at the end of the process.
The inquiry has been transferred to the Met's specialist crime directorate and will be led by Deputy Assistant Commissioner Sue Akers.
'Crisis of trust'
Acting Deputy Commissioner John Yates told authority members that a new inquiry had not been opened before now because none of the information was new to police.
"I was being asked to act on rumour, innuendo and gossip. I have always said we will respond to any new evidence and that is exactly what we have done today," he said.
He went on to explain that the police could not contact potential victims to provide them with information on the case for civil purposes without a court order.
But a source on one of the legal teams acting for those who believe their phones have been hacked told BBC political correspondent Ross Hawkins they disputed this assertion.
Meanwhile, the former chairman of the Lords' communications committee, Lord Fowler, has called for a "full scale inquiry" into the case, while former Deputy Prime Minister Lord Prescott has restated his demand for a judicial review.
Lord Fowler said: "We need to know what techniques were used, we need to know how widespread they were, and above all how the public can be protected. That's the issue at the centre of this."
Former Scotland Yard assistant commissioner Brian Paddick, who believes his phone was hacked into by another newspaper, accused the force of running scared of the press.
And government whip Lord Wallace of Saltaire said the press faced a "crisis of trust" comparable with that faced by MPs following the parliamentary expenses scandal.
A growing number of public figures have launched civil legal actions against both the NoW and the police amid allegations the practice of phone hacking was widespread.
The latest to come forward on Thursday include:
- Shadow Cabinet Office minister Tessa Jowell, who said she had informed police about an apparent attempt to access her voicemail last week. Her phone company contacted her to say there had been an unsuccessful attempt, but she is not sure if journalists were behind it or not
- Actress Leslie Ash and her ex-footballer partner Lee Chapman, who are preparing legal action over fears their phone messages were intercepted while she battled a hospital superbug.
Scotland Yard's decision to reopen the hacking investigation follows a NoW internal inquiry that led to the sacking of its assistant editor (news), Ian Edmondson, on Tuesday.
Legal advice from the Crown Prosecution Service (CPS) in the past has been that phone hacking was only against the law if the message had not already been heard by the intended recipient.
That was never tested in court, and it has been confirmed to the BBC that the CPS has changed its advice so that accessing old messages would also be illegal.
BBC business editor Robert Peston has learned that News International, which owns the paper, uncovered four e-mails showing that Mr Edmondson had full knowledge of illegal phone hacking. The details were passed to police.
Meanwhile, the BBC has seen documents relating to the hacking of a phone owned by interior designer Kelly Hoppen, allegedly by reporter Dan Evans, who was suspended from the paper last year.
She had successfully won a court order forcing a telephone company to release the identity of anyone allegedly trying to hack her phone between June 2009 and March 2010.
Both Ms Hoppen and her stepdaughter, the actress Sienna Miller, are taking action against the newspaper. News International and Mr Evans are defending the allegations.
In a statement NoW said: "We have carried out an extensive investigation led by a team of independent forensic specialists and we have found no evidence whatsoever to support this allegation.
"The civil litigation is ongoing, as is the internal investigation and until both are concluded it would be inappropriate to comment further.
"However, we are disappointed the BBC chose to lead with this misleading report without giving the News of the World an opportunity to respond."
The Guardian has previously reported that Mr Evans' defence is he phoned Ms Hoppen's number for legitimate reasons and accidentally accessed her voicemail when the keys on his own phone got stuck.
Mr Edmondson was suspended from active duties last month after he was identified in court documents as having instructed private investigator Glenn Mulcaire to access phone messages.
Mulcaire was jailed for six months in January 2007 alongside royal editor Clive Goodman, who was sentenced to four months, for hacking into the mobile phones of royal aides.
Last week Prime Minister David Cameron's official spokesman Andy Coulson resigned, saying the media storm surrounding ongoing hacking claims had distracted him from his work.
Mr Coulson edited the NoW from 2003 to 2007 and resigned from that job following the convictions of Goodman and Mulcaire.
However, he has always denied having any knowledge of hacking, and a source close to him has told the BBC he is not implicated in any way by the new evidence that has come to light.