The police watchdog is investigating claims that a Surrey Police officer gave information about the Milly Dowler murder probe to the News of the World.
The Independent Police Complaints Commission (IPCC) said it received a voluntary referral from the force about an allegation that an officer gave information to the newspaper in relation to the investigation into Milly's murder in 2002.
The watchdog said a decision had been made to investigate independently.
A statement said: "The IPCC has received a voluntary referral from Surrey Police regarding information they received alleging that a Surrey officer gave information to The News of the World in relation to the investigation into Milly Dowler's murder in 2002.
"A decision has been made that the IPCC will independently investigate.
"An IPCC Deputy Senior Investigator has been over at Surrey Police to get more information about the case and will be writing to the Dowlers' solicitor at the family's request.
"Until we have shared further detail with him it would not be appropriate for us to make it public."
The probe comes just weeks after revelations that a detective was removed from the case for leaking information.
Last month Surrey Police confirmed the unnamed officer was "given words of advice" and permanently taken off the probe in 2002 for telling a friend, a retired police officer, details about the investigation.
At the time, a spokeswoman said there was no suggestion of any officer sharing information with The News of the World.
She said a serving colleague reported the officer after they were told about the "inappropriate disclosure" by the person who had heard it.
At the time of the revelation, the Dowler family's solicitor Mark Lewis said the family were "distraught that such matters were taking place when the police were supposed to be looking for their daughter".
Of the IPCC's announcement, he said: "The Dowler family welcome the full investigation into what happened at the time of the initial inquiry into Milly's disappearance.
"It would not be appropriate to make any further comment at this stage."
A News International spokeswoman said: "News Corporation's independent Management and Standards Committee is co-operating fully with the Metropolitan Police and is facilitating their investigation into illegal voicemail interception and related issues at the News of the World."
A spokeswoman for Surrey Police said: "Surrey Police has referred a matter relating to the conduct of an officer involved in the Operation Ruby investigation to the IPCC.
"Any further inquiries should be redirected to them."