Surrey Police was aware that Milly Dowler's phone was hacked by someone working for the News of the World (NoTW) in 2002, the force's chief has said.
Chief Constable Mark Rowley said the now-closed Sunday newspaper made a call to the police operation room coordinating the inquiry into the schoolgirl's disappearance in April 2002 which made it apparent it had accessed her voicemail.
But officers only "focused on retrieving any evidence the NotW had that could assist in the investigation into Milly Dowler's disappearance" as that was the priority, Mr Rowley said.
No criminal investigation was launched into how the News International newspaper came by the information it provided and Surrey Police "neither arrested nor charged anyone" in connection with the hacking, he added.
In a letter to the Commons Home Affairs select committee, Mr Rowley said an inquiry was under way into why this was the case.
He added it appeared his force also failed to pass this information on to the Metropolitan Police's original phone hacking investigation in 2006.
Keith Vaz, the committee's chairman, said: "Had Surrey Police acted in 2002, it may have prevented the culture of hacking becoming endemic at News of the World. This was a serious omission. The committee will be investigating further the reasons why Surrey Police did not follow up on this evidence, and why Sussex Police did not flag it up in their review of Operation Ruby."
In his letter, Mr Rowley said: "From that call it was apparent that person(s) working for, or on behalf of, the NotW had accessed Milly Dowler's voicemail."
Mr Rowley added: "At that time, the focus and priority of the investigation was to find Milly, who had then been missing for over three weeks and significant resources were deployed to achieve this objective.
"I can confirm that Surrey Police did not launch a criminal investigation into how the NotW came by the information it provided Operation Ruby with in April 2002 and that Surrey Police neither arrested nor charged anyone in relation to accessing Milly Dowler's voicemail. The inquiry team is currently looking into why this was the case."