Scotland Yard's phone-hacking squad is working its way through 300 million emails from News International, the force's chief revealed today.
Commissioner Bernard Hogan-Howe said police had already spent up to £3 million on salaries, with officers speaking to 1,800 of 6,000 potential victims.
Mr Hogan-Howe said the "scale of the task is pretty large" as it emerged a total of 120 officers and staff are now working on the investigation.
But he added that the investigation would not be rushed.
Speaking at New Scotland Yard, he said: "You either do it properly or you do it quickly."
Referring to criticism of the first investigation into illegal activities at the News of the World, he said: "It is best that we do it thoroughly."
When asked about cost, Mr Hogan-Howe said there was an issue "for society" in how far the Metropolitan Police pushes the investigation.
But he added: "We have now got over 100 people working on this case which just shows the scale of it and seriousness with which we are taking it."
When asked whether evidence of phone-hacking had been discovered at other newspaper groups, he said: "We will follow the evidence."
A total of 1,800 victims have come forward to speak to police.