The police chief leading Scotland Yard's phone hacking probe has revealed just 170 of more than 4,000 potential victims have been contacted.
The full scale of the long-running operation was laid bare as Deputy Assistant Commissioner Sue Akers admitted "confidence has been damaged" by previous failures.
She admitted there was an "awful lot to do" after saying police had compiled a list of more than 12,000 names and numbers.
Evidence from News International details 3,870 names along with 5,000 landline numbers and 4,000 mobiles, she told the committee of MPs.
She gave MPs her "guarantee" that she would oversee a thorough inquiry, adding: "I hope that I do not have come back here in five years time."
Ms Akers said confidence had been dented after criticism and analysis of the previous operation.
She added: "I do not doubt that if we do not get this right it (confidence) will continue to be damaged."
Referring to the number of victims contacted, she added: "There's an awful lot more work to be done, I can see how that looks."
More than 11,000 pages of material uncovered during the inquiry, known as Operation Weeting, are being uploaded on to a searchable database in a bid to speed up the inquiry, Ms Akers added.
Ms Akers gave evidence after a string of police chiefs were quizzed over their efforts by the Home Affairs Select Committee.
Metropolitan Police Assistant Commissioner John Yates and former commissioner Lord Blair confirmed they were among the list of potential targets at the News of the World.
Mr Yates said he was "99% certain" his phone had been hacked while Lord Blair, Britain's former top policeman, told the committee: "What I am aware of is that my mobile and home telephone numbers were within the files that have been examined."
Mr Yates expressed regret at his 2009 ruling that there was no need to reopen the phone-hacking investigation.
But he insisted he had always told the truth to the Home Affairs Select Committee and suggested that the News of the World "failed to co-operate" with police until the start of this year.
"I can assure you all that I have never lied and all the information that I've provided to this committee has been given in good faith," he told the MPs.
Mr Yates strongly denied allegations in the New York Times that he was put under pressure not to investigate phone hacking at the News of the World because of fears that the Sunday tabloid would publish details about his personal life.
Labour MP Mr Vaz told Mr Yates at the end of the session that the committee found his evidence "unconvincing".