Police investigations into phone-hacking, privacy breaches and corrupt payments could cost £40m over four years, the officer in charge of the inquiries said today.
Deputy Assistant Commissioner Sue Akers told the Commons Home Affairs Select Committee that while "an enormous amount of money" is being spent on the probes, she is not concerned by the fact that only just over half the possible victims have been notified and 13 people charged.
So far, eight people including former News International chief executive Rebekah Brooks and David Cameron's ex-spin doctor Andy Coulson have been charged under Operation Weeting, which is looking at alleged phone-hacking.
This is running alongside Operation Elveden, an inquiry into accusations of corrupt payments to officials, and Operation Tuleta, which is looking at allegations of computer hacking and other privacy breaches.
Brooks, her husband Charlie and four other people have also been charged with conspiring to pervert the course of justice over allegations that they removed boxes of material from the News International archive and tried to conceal documents, computers and other material from police.
Of a total of 4,744 potential and likely victims of phone hacking, only 2,500 have been notified, and the rest are deemed non-contactable, Ms Akers said.
She was asked by committee chairman Keith Vaz: "£40 million of taxpayers' money, half the people haven't actually been notified, only eight people have been charged, is that a concern to you?"
Ms Akers replied: "No. I think that the fact that people have been charged under Weeting represents a success in our investigation. Elveden is still under consideration."
The investigation team has identified "potential" victims as those whose name and phone number appears in the material police are examining.
"Likely" victims are defined if there is other material present, for example a voicemail PIN or audio tapes or transcripts.
As of August 31, there were 3,675 potential victims of whom 1,894 have been contacted, and 1,781 who could not be reached.
There were 1,069 likely victims, of whom 658 have been contacted and 388 were not contactable. Officers decided not to tell the remaining 23 people for operational reasons.
Ms Akers said: "We're dealing with material that is six years old, so lots of people don't have the same telephone numbers, people move on.
It's very difficult this long after the event. There's a whole range of reasons why we haven't been able to contact them. We have to draw the line somewhere."
The total number of arrests made so far is 25 under Operation Weeting, 43 under Elveden and 11 under Tuleta.
The three operations have cost £9 million this year, and it is predicted that the total cost will be £40 million over four years.
Ninety-six officers are working on Operation Weeting, 70 on Elveden and 19 on Tuleta.
Akers said the force will be under financial pressure after the Olympics, with the Met having to cut millions of pounds from its budget, and it will be hard to decide when to end the investigations.
She said: "An exit strategy is one of the most difficult issues. In terms of the phone hacking it's perhaps easier to see an end because we now have people charged. That needs to take its course through the courts.
"In terms of the corrupt payments that very much depends on the co-operation of the papers. If we're uncovering corrupt police officers we feel that we should continue to do that. But at some point – there is an enormous amount of money being spent on this, a lot of police resource and post-Olympics we're going to be in very tight Financial Times."
The team have to sift through 12 terabytes of data, equivalent to one million phone books, Vaz said.
Ms Akers said that when she retires, Deputy Assistant Commissioner Stephen Kavanagh will take over her role leading the investigation.