A private detective working for the News of the World allegedly hacked into Milly Dowler's mobile phone after she went missing, a lawyer for the murdered schoolgirl's parents claims.
Journalists at the Sunday newspaper are also accused of deleting some voicemail messages on the phone in the first days after she vanished, the Guardian reported.
Solicitor Mark Lewis described the alleged actions as "heinous" and said they offered Bob and Sally Dowler "false hope" that their daughter was still alive.
Scotland Yard officers contacted Mr and Mrs Dowler about the allegations in April, a month before Levi Bellfield went on trial for her murder.
News International, the publisher of the newspaper, said the allegations were of "great concern" and said it would be conducting its own inquiry.
Private investigator Glenn Mulcaire is alleged to have illegally accessed Milly's phone messages after she was abducted by Bellfield as she walked home from school in Walton-on-Thames, Surrey, in March 2002. The voicemail messages were allegedly deleted to make room for more after Milly's inbox became full.
Mr Lewis, from London-based Taylor Hampton Solicitors, said: "Sally and Bob Dowler have been through so much grief and trauma without further distressing revelations to them regarding the loss of their daughter."
Responding to the Dowler family allegations a News International spokesman said: "We have been co-operating fully with Operation Weeting since our voluntary disclosure in January restarted the investigation into illegal voicemail interception.
"This particular case is clearly a development of great concern and we will be conducting our own inquiries as a result. We will obviously co-operate fully with any police request on this should we be asked."
Labour leader Ed Miliband said: "I am shocked by the news of the hacking of Milly Dowler's phone. It beggars belief that anyone would undertake such a cruel and immoral act. The police inquiry must get to the bottom of who was responsible for this and who was complicit in it."