The police watchdog is investigating an allegation that a Surrey officer gave information about the Milly Dowler murder investigation to the News of the World.

The Independent Police Complaints Commission (IPCC) said it was investigating the claim following a voluntary referral from the force. "An IPCC deputy senior investigator has been over at Surrey police this morning to get more information about the case and will be writing to the Dowler family, via their solicitors at their request, this afternoon and offering to meet to give them more detail," the commission said. "Until then, it would not be appropriate for us to make any further detail public."

The Guardian understands that the allegations relate to the early stages of the investigation into Milly Dowler's disappearance.

It is thought a Surrey police officer met a female journalist from the News of the World at a social event in London and told her details about the leads that officers working on the case were following. It is not thought that he was paid for the information.

Sources claimed the officer in question was publicly admonished in front of colleagues when the paper subsequently published the information in a story, taken off the case, and then disciplined.

The Dowler family's solicitor, Mark Lewis, said he had not been told how long the IPCC investigation will take.

"They need to look at their own information about who the officer was, what he said and what he gave out," he told the BBC News channel, referring to the information passed voluntarily to the commission by Surrey police.

The revelation last month that a mobile phone belonging to the teenager was accessed by the News of the World triggered a wave of public revulsion that led directly to the closure of the 168-year-old paper.

The Dowler family received a personal apology from Rupert Murdoch, chairman of the paper's owner News Corp. They also met with the prime minister.

Lewis said the family were upset to learn a policeman may have passed information to the paper during the original 2002 investigation.

He said Milly's parents had already endured the trial of Levi Bellfield, who was convicted this year of murdering Milly, during which they were cross-examined aggressively by Bellfield's lawyer.

That was followed almost immediately by allegations that Milly's voicemail messages were listened to by the paper and then deleted in order to create room for more messages to be left.