Deputy Assistant Commissioner, Sue Akers, told the Home Affairs Select Committee that Scotland Yard had identified 4,744 potential or likely victims.
Of those 2,500 had been informed by the police, while the others were either not traceable or had not been informed for "operational reasons".
In her last appearance before the Committee before she retires from the Metropolitan Police, DAC Akers said Operation Weeting - set up to investigate phone hacking - was drawing to a conclusion.
All those who are contactable have now been informed by the police and the operation is being wound down. So far eight people have been charged with conspiracy to hack phones while the CPS are examining a number of other files.
Explaining the progress that had been made DAC Akers said the number of potential hacking victims - those whose names and telephone numbers were recovered during the investigation - stood at 3,675, of which 1,894 had been informed.
The number of likely victims - those for whom more information had been recovered by police - stood at 1,069. Of them 658 had been contacted, 388 were not contactable and 23 had not been contacted for "operational reasons".
She explained that 25 people had so far been arrested as part of Operation Weeting, 12 of whom are current or former journalists.
DAC Akers said there were currently 96 police officers working on Operation Weeting, while 70 were employed on Operation Elveden, examining allegations of corrupt payments to public officials and 19 were working on Operation Tuleta, looking at computer hacking.
The cost of the overall investigation for the last year was £9 million, but the projected cost over the next four years was put at £40 million.
DAC Akers said while Operation Weeting was drawing to a conclusion, the other two investigations were still ongoing. Asked whether the cost of the investigations could still be justified, she admitted it would be a question that would need addressing at some point.
She told the hearing: "There is an enormous amount of money being spent and a lot of resources being used. Post Olympics we are going to be in very difficult financial times."
When she appeared before the Leveson Inquiry into press standards DAC Akers revealed that as well as News International, the Met were investigating allegations of corrupt payments from journalists working at Mirror Group and Express Newspapers.
Asked to say whether any other newspaper organisations were now being investigated, she declined to say.
