A police watchdog is investigating allegations that a senior officer knew murdered teenager Milly Dowler’s voicemails were illegally accessed by the News of the World.

Surrey Police Authority voluntarily referred the potential conduct matter, relating to Deputy Chief Constable Craig Denholm, to the Independent Police Complaints Commission (IPCC) on Thursday, June 21.

Deputy Chief Con Denholm was the senior investigating officer for part of Operation Ruby, the Surrey Police investigation into the abduction and murder of Milly Dowler in 2002.

The freshly-launched IPCC investigation will consider whether Deputy Chief Con Denholm was aware, during Operation Ruby, that the newspaper had accessed Milly Dowler’s voicemail and look at how he handled that information.

A referral was also received from Surrey Police on the same day, June 21, in relation to the conduct of Temporary Detective Superintendent Maria Woodall, who was a Detective Chief Inspector and the senior investigating officer on Operation Ruby from 2006 onwards.

The IPCC investigation will examine the information she provided to Surrey Police during the course of the internal inquiry into the force response to allegations that Milly Dowler’s voicemail had been illegally accessed in 2002.

In July 2011, Surrey Police launched an internal investigation – Operation Baronet – to look into circumstances surrounding the Walton teenager’s voicemails.

The Baronet team, under the leadership of Assistant Chief Constable Jerry Kirkby, is providing ongoing support to the Met’s hacking investigations, Operations Weeting and Elveden, and is preparing to report to the Leveson Inquiry.

Under Operation Baronet, Surrey Police voluntarily referred a separate potential conduct matter relating to Temporary Detective Superintendent Maria Woodall.

In a statement, the Dowler family said: “The Dowler family welcomes the proper investigation of what happened at Surrey Police 10 years ago.

“They regret that the passage of time means that some individuals can now no longer be investigated. The family have no further comment to make at this time.”

A spokesman for Surrey Police and the force authority said: “Surrey Police Authority and Surrey Police believe that seeking independent scrutiny into these matters at this stage is the appropriate course of action.

“The IPCC will have our full co-operation and support throughout their investigations.”